BLASTOPOEB, MBSOBEEM AND METAMESIO SEGMENTATION. 15

Blastopores Mesoderm and IffetamerieSegmentation.

By

W . M. Caldwell ; , H.A. ,fellow of Caius College, and Balfour Studeat in the University of Cambridge.

With Plate II.

A TEAK ago my observations on the development of certain,forms of invertebrate animals suggested an explanation, of t iebehaviour of the blastopore, and led me to consider the variousspeculations concerning the middle germinal layer put forwardin recent years. The theory which I thus deduced embraces thequestion of metameric segmentation. On my voyage to AustraliaI wrote the present paper. Meanwhile my friend Mr. Sedgwiek,to whom I had written some of my conclusions, was preparinga contribution to the same subject. Since then Mr. Sedgwick'spaper1 has appeared, and I find not only that my conclusions asto the meaning of segmentation are fundamentally different;but also that he leaves the larger question of mesoderm un-touched. The origin of mesoderm in Phoronis was the starting-point of my inquiry. I shall first describe the facts in thisanimal, and I would point out that my discoveries are dueentirely to the facilities for investigating minute embryosafforded by my method of obtaining automatic series of sections.I failed to observe the details which are contained in the presentpaper in my original sections from which a preliminary account{' Proc. Eoy. Soc.' 1882) was composed.

! ' Quart. Journ. Mic. Soi.,' 1884, " Origin of Metameric Segmentation."

16 W. H. CA1DWELL.

DEVELOPMENT OF PHOKONIS.

The details of the segmentation of the ovum are not requiredin this paper. A planula slightly oval in form is the final termof the process. The long axis of this planula coincides withthe future long axis of the gastrula. One half of the cells arelarge (endoblast), the other half are small (ectoblast).

GASTRULATION.

The gastrula is formed by invagination. The first sign ofthis is the flattening of the endoblast-half of the oval planula.The sides begin to grow over the endoblast, and this takes placein such a way that the saucer-shaped structure is deepenedtowards the future anterior end (PL II , fig. 1). The anteriorend also grows rapidly over the endoblast, thus early indicatingthe future prseoral lobe. The bilateral symmetry is thusclearly marked. Posteriorly the sides fold over so as to meet inthe middle line (PI. II , fig. 9). The cavity of the archenteronis now sufficiently large to form lips to the blastopore. Quiteposteriorly the lips completely fuse, so that during the gastru-lation the extreme posterior portion of the archenteric cavity isobliterated (PI. II, fig. 10). It is represented by a fused solidmass of cells (/•_). The lips of the blastopore continue toapproach the middle line, and as they touch fuse with eachother. This fusing proceeds from behind forwards. Theblastopore has in this way become divided into two partsexactly comparable to the parts long known in some vertebrates(PI. I I , fig. 2). I shall speak therefore of the posterior portionas the primitive streak, and the groove along the line of closureas the primitive groove. The invagination has now produceda gastrula with an opening situated in the anterior portion ofthe blastopore into a large archenteric cavity (PL I, fig. 11). Ishall now use the terms dorsal and ventral as defined by thenon-blastoporic and the blastoporic regions respectively. Theventral surface now begins to grow very much more rapidlythan the dorsal. The growth results in the posterior point ofthe primitive streak becoming terminal. The exact* behaviour of

BLASTOPOEE, MESODERM AND METAMEEIC SEGMENTATION. I ?

the cells of the middle of the primitive streat is as follows.About the middle of the primitive streak the ectoblasticelements divide very rapidly, and very soon the primitivegroove disappears in this region (si. PL II , figs. 2—6). Coinci-dently with this a space appears between ecto- and endoblast(PL II , fig. 18, v). Consider the fate of a single one of thecells of the primitive streak. This ceil is destined to give riseby division to both ecto- and endoblast. The ectoblastic por-tion increases more rapidly than the endobiastic, and soon thelatter is no longer in contact with the former, i.e. a space hasarisen in the median ventral line. This space extends bothanteriorly and posteriorly. The primitive groove BOW onlyremains as a pit at the posterior end of the embryo (fig. 4, g).The anterior opening of the blastopore remains open andbecomes the month of the future Phoronis, the posterior pitis destined to undergo some very remarkable changes whichwill be described below. During these changes the originalsolid mass of cells—the posterior portion of the primitivestreak—remains unaltered as a cord connecting the archenteronwith the ectoderm.

THE SEPARATION OF THE MESOBLAST.

Previous Observations.—Kowalewsky originally de-scribed the mesoblast as originating in Phoronis by delaminationfrom the ectoderm (vide Plate II , fig. 13). This mistakearose from the ectoderm cells being darker at their base.Recently Metschnikoff and Foettinger have attempted to solvethe problem of mesoderm formation. While they have bothrecognised Kowalewsky's error, they have fallen into othermistakes. Metschnikoff describes some mesoblastie cells alreadypresent in the blastula stage; he figures four of them in Ms fig.30. In each cell Metschnikoff has drawn a nucleus. I have fre-quently observed this appearance of cells. It is caused, how-ever, by the amoeboid processes of the endoderm ceils growi%into the segmentation cavity. This is easily proved by makingreal sections. Another possible explanation of Metschnikoff'saccount may lie in the presence of certain peculiar bodies in

VOL. XXV. NEW SEE. B

18 W. H. CALDWELL.

the early gastrula stages. In the endoderm. cells little sphericalmasses of apparently the same material as the body of the cellsthemselves are frequently found. Each little mass in hardenedembryos is separated off by a clear space. I have traced thesebodies from their birthplace into the body cavity. Theynever possess a nucleus, and they disappear at a very early age.Their significance remains unknown to me unless they bemerely an excess supply of nutriment analogous to food yolk.Foettinger has arrived at somewhat extraordinary results. Hesays, " J'ai non seulement constate 1'existence des premierselements-mesodermiques a des stades plus jeunes que celuisignale par Metschnikoff, mais encore je crois pouvoir reculerleur premiere apparition jusqu'a Fceuf en voie de segmenta-tion." The bodies referred to the mesobiast are, I believe,either due to the reagents used in preparing the embryos, orare the bodies referred to above. I have observed them fre-quently, but it is certain that they have nothing to do withthe true mesobiast, whose origin I shall now describe.

Before the lips of the blastopore meet there is no meso-biast (fig. I). When the closing of the blastopore has alreadyextended sufficiently far forwards to shut off a small archen-teric cavity, two pouchings of the endoderm occur on eitherside of the blastopore (fig. 8, ad). Each pouch is longi-tudinally extended in the direction of the long axis of thebody, and is deeper towards its anterior end. The endodermcells covering the region of the pouch now undergo somedivision (fig. 8), and a mass of cells is budded off on eitherside {me'). These cells as they are formed arrange them-selves into a sac enclosing a cavity (fig. 17, ad). These cavities,however, never communicate with the cavity of the gut.The pouch of the endoderm is soon obliterated, and thecells return to the size of the other endoderm cells. Thehind part of each pouch lies about opposite to the most anteriorpoint where the lips of the blastopore have closed. Oneither side of the primitive streak a few mesobiast cells arebudded off from the cells forming the primitive streak (fig.18, me"). Behind the primitive groove becomes deeper, and

BLASTOPOKE, MESODEEM AND META3TERIC SEGMENTATION. 19

this deepening continues after the middle part is obliterated(figs. 4, 6, 11, and 12). The deepening of the groove soonforms a very definite pit {g). This pit, when by the growthof the ventral surface it has become nearly terminal,, growsinto two pouches which project into the caYity between theskin and gut on either side of the solid cord of cells, whichis the persistent hind part of the original primitive sheath(fig. 13 and fig. 14, pd). These pouches are derived fromcells homologous with those which have already given riseto mesoblast. The continuity of this posterior pair of pitswith the anterior is kept up by the few cells (me") buddedoff in the middle of the primitiTe streak. The same growthwhich opened up the space between ectoblast and endoblasthas separated the anterior and posterior mesoderm. The factthat in Phoronis the two ends of each mesodermie pouch areactually connected by an intermediate cord of cells depends onthis formation of a primitive streak along the whole line ofclosure of the blastopore.

NEPHEIDIA.

The posterior pair of mesodermie diverticnla open in themiddle line to the exterior. The closure of this opening pro-ceeds in such a way that each pouch remains open to theexterior by a small pore on either side of the middle line. Ibelieve—though this fact is not established so certainly as thoseabove concerning the mesoderm—that each pore persists asthe opening of the nephridium of its own side. The nepitridiaappear eoincidently with the final narrowing of the mesodermicpores, but I have yet no sections showing the cells in theneighbourhood of the pores taking on the form of the Intra-cellularly perforate excretory cells. The formation of theseexcretory cells, which lie in a blood space of the splanchno-pleure and not in the body cavity, I have independently tracedfrom the mesodennic cells of the posterior pouches.

20 W. H. OALDWELL.

ANUS.

Meanwhile the remnant of the primitive streak, the posteriorsolid cord of cells, opens up, and forms a canal leading fromthe archenteron to the exterior (fig. 15). The alimentary-canal is now complete, mouth and anus having been derivedfrom the blastopore (fig. 7, m and a).

HYPOTHESIS.

With the help of the various morphological laws impliedby such terms as precocious segregation, superlarvation, abbre-viation, &e., it is possible to solve almost any morphologicalproblem in several ways—all equally probable. The speculationswhich follow, I am induced to add to those already existing,not from any belief in their absolute value, but because theygo in the direction of simplification. The theory which I amabout to state reduces the various origins of the mouth andanus to one type. The same hypothesis gives an explanationof the various modes of origin of the mesoderin, and leads to aview of the meaning of metamerie segmentation which, so faras I knoWj has not been hitherto suggested.

Given a gastrsea already become bilaterally symmetrical bythe elongation of the blastopore and the differentiation ofanterior inhalent and posterior exhalent currents, and in whichthe main development of organs takes place around the mouth,so that the mesoderm thus resulting comes to lie in develop-ment as two masses of cells on either side of the body.1

I propose to show that the elongation of a long axis of thebody is a possible cause of—

I. The obliteration of the relation of blastopore to mouthand anus.

II. The masking of the original mode of mesodermic forma-tion.

III. Metamerie segmentation.1 Whether the mesoderm originally arose as diverticuia or not does not

concern the present speculation.

BLASTOPORE, MESODERM AND METAMERIC SEGMENTATION. 21

I. The Obl i te ra t ion of the Rela t ion of Blas toporeto Mouth and Anns .

Previous Observations.—Since the time of the GastraeaTheory many writers have occupied themselves with the blas-topore. Lankester (This Journal, 1877), in accordance with, hisplanular theory, came to the conclusion, that the coincidence ofmouth and aaus with the blastopore was only a developmentalconvenience. He says (loc.cit.), "Regarding, as I do, the blas-topore as an orifice of a secondary nature, esisting solely in rela-tion to the invagination process, and originating after mouthand anus had made their appearance in the progress of animalevolution, I seek to explain its occasional relation to the mouthand to the anus as cases of adaptation." Balfour, after enu-merating the different fates of the "blastopore in the animalkingdom, says, " It is clearly out of the question to explain allthese differences as having connection with the characters ofancestral forms. Many of them can only be accounted for assecondary adaptations for the convenience of development."The number of groups in which a slit-like blastopore has beendescribed is very considerable (vide Balfour, vol ii, p. 282).Lankester was the first to suggest that a slit-like blastoporewhich might close at either end would, if taken as the ancestraltype, account for the various fates of the blastopore in mol-luscs. Hatschek, in his paper on Teredo, has suggested thepossibility of phylogenetically deriving the anus which arisesin this animal; secondarily, as an ectodermic invagination frompart of a slit-like blastopore. But he bases this view on the factthat the anus corresponds in position with the hind wall of the.gastrula mouth. Metschnikoff, in combating Hatsehek's viewson the early expression of bilateral symmetry, has denied theancestral character of the slit-like blastopore. He says, " Kaiinman dem gesehlitzten grossen Biastopor keine paiingenetiseheBedeutung zuschreiben und muss ihn als eine embiyonaleAnpassungs-erschehrangansehen." Sedgwick (loc. cit.,p. 27)concludes that " the mouth and anus of the Triploblastica are

22 W. H. CALDWELL.

derived from the primitive mouth." I fail to see how his theoryon p. 34 explains the behaviour of the blastopore. In the firstplace his conception of the blastopore is diiferent from that usedin the present paper. Page 35 he writes, " Consequently the onlycourse open is that the mouth should be formed as a secondaryperforation entirely independent of the blastopore." Sedg-wick's theory is contained in the following passages (p. 34):" My view is that in those animals in which it does not giverise to the mouth and anus, it functioned as the larval mouthwhile the animal was developing, and persisted until parts ofthe embryo were developed between it and the position of themouth and anus of the adult, which parts had arisen in thephylogenetic history in the adult after the primitive mouth hadcompletely divided into the mouth and anus. These partsnever had. been traversed by the original slit-like mouth,because they had. appeared at a stage in evolution subsequentto the stage in which the mouth and anus were one. It cannottherefore be a matter of surprise if the blastopore does notelongate and bisect these latter structures, which never had inthe history of the animal been perforated by the blastopore."I ask, how have the cells which are to form mouth andanus anything to do with blastopore ? My hypothesis is asfollows.1

The behaviour of the blastopore in Phoronis is obviously dueto the attainment of a terminal anus. Suppose the long axisof the body to increase still more rapidly while the posteriorpart of the blastopore still remains terminal. Suppose in theearly stages of development the importance of a completealimentary canal is not equal to the importance of the bodyform, then the tendency of the endoblast to divide into anteriorand posterior portions attached to anterior and posterior partsof the blastopore respectively might be consummated. Thebehaviour of the cells in the middle of the primitive streak ofPhoronis, which resulted in the opening of a space betweenendoderm and ectoderm, would tend to begin at an earlier stage.

1 Delamination need not be discussed, since the existing hypotheses (videBalfour, 'Embryology,' vol. ii) are sufficient to bring the case under my theory.

BtASTOPOBE, MESODEEM AXD METAMEEIC SEGMENTATION. 23

What would be the structure of a pianula when this influencehas reached Tback to the preinvaginated stages ?

The ectoderm cells would grow in from either side, and en-croach on the solid mass of endoblast until this (i. e. endoblast)would be completely divided into anterior and posterior masses.The endoblast may be divided in this way into any number ofseparate portions (ef. below metameric segmentations). Letinvagination now take place; each mass of endoblast will beinvaginated. The invaginated endoblast will grow accordingas it has been divided into endoderm or mesoderm; suchdivision may have taken place in a great many ways (videmesoderm below); when the anterior endoblastie mass is thelarger we have a so-called oral blastopore, e. g. Pilidiuin,Phascolosoma, and Phoironis, according to previous observers;when the posterior is the larger we have an anal blastopore,e. g. Paludina, Serpula, and Echinodermata. Let the invagi-nation of the different endoblastie masses cease to be syn-chronous, and the primitive relations will become still moremarked. The extreme cases are described as stomodosa andproctodcea. The difference between Sedgwiefs view and myown consists in the fact that I suppose that portions of the bias-topore actually exist beyond the " parts which, in the phy-logenetic history in the adult, had arisen after the primitivemouth had completely divided into the mouth and anus."

II . The Masking of the Original Mode of the Originof Mesodermic Formation.

In Phoronis the original pair of diverticula are almost dividedinto two pairs. The anterior pair produce only a small proportionof the future mesodermic structures. Argiope is an instance ofthe anterior mesoderm being large. No posterior mesoblast hasyet been described in this form. The mesenehyme of the preorallobe of the Hertwigs is the same anterior mesodermic diverticulareduced in the oppositedirection. In Phoironis the anterior meso-derm would be described as of endoblastie origin, the middle asoriginating at the lips of the blastopore, while the posteriorpouches would be assigned to the ectoblast. But the connec-

24 W. H. CALDWELL.

tion between these three methods, though obvious in Phoronis,has not been explained in the same way in the rest of theTriploblastica. The six diagrams (fig. 19) represent six dif-ferent modes of formation; they may all coexist in the sameanimal. Thus, in Phoronis we find those represented in thediagram by 3, 4, and 6, and in the Chick and other Verte-brates 4 and 6.

III . Metameric Segmentation.

In Phoronis the elongation of the blastopore produces twopairs of masses of mesoblast, each of which might be re-garded as constituting a " mesodermic somite;" but in Phoronisthe first long axis developed is not to be the long axis of theadult. The long axis of an adult Phoronis is exactly at rightangles to that of the larva. The further slight extension ofthe larval long axis is thus able to proceed pari passu withthe growth of the posterior pair of mesodermic pouches. InChaetopoda, Arthropoda, and Vertebrata the long axes of adultand larva are identical. The elongation of the body in theseforms takes place before the mesoderm grows. The samecause which separated the mesoderm in Phoronis operatesduring a much longer developmental period. The mesodermcannot keep pace with the ectoderm. It must therefore beleft to afterwards complete its growth. The various positionsin which it may remain give rise to the various origins of themesoderm. Take Amphioxus, where the mesoderm has remainedentirely in the endoblast. Here we have a regular elongationof the body taking place when the mesodermic cells are stillundifferentiated. The mesodermic diverticula are regularlydrawn out, and as regularly they leave small portions of thewhole in front. Hatschek has described a shallow grooveconnecting the separated diverticula on each side, which isexplained by the present hypothesis. I take it to be of thesame nature as the connecting strand of mesoderm in Phoronis.Echiurus (Hatschek) is a form where the greater part of themesoderm remains near the posterior pole of the long axis as inPhoronis. As the long axis grows the mesoblast has to be left

BLASTOPOEE, MESODEEM AND METAMERfC SEGMENTATION. 25

in formative masses, which afterwards grow to line the bodycayity (in Echiurus, however, the mesoderm much more nearlykeeps pace with the eetoderm than in Amphioxus). Thus, Iconsider that in Cfaaetopoda, Arthropoda, and Vertebrata themesoderm will tend to be left in regular pairs of masses, whilethe elongation of the body is taking place; I seek to explainthe whole of the facts of metameric segmentation as arisingfrom the necessities of development.

In Phoronis the external openings of the nephridia are partsof the blastopore. The same considerations which have beenapplied to the mouth and anus, mesoderm and metamericsegmentation bear on the question of the origin of nephridia.The nephridial portions of the mesoderm may remain invarious positions, in other words, the nephridia may in accord-ance with my hypothesis arise as single or as serial ectoblasticor endoblastic pairs of pouches with or without connectinglongitudinal canals or cords.

Instances of most of these possible modes of origin havebeen described in the different groups of animals. The bearingof the above facts and hypotheses on the nervous system, gill-slits, notochord, and other organs, will be obvious to anyonewho has followed me so far. I hope to be excused fromentering into completer discussion of my hypothesis by reasonof the want of books of reference in my present situation outof the world.

SUMMARY.

Facts in the development of Phoronis—1. The blastopore gives rise to both mouth and anus.2. The mesoderm arises in an anterior pair of endobiastie

modified diverticula, and in a posterior pair of eetoblasticdiverticula connected by a few mesodermic cells derived fromthe middle of a primitive streak.

3. The nephridial openings to the exterior are parts of theblastopore.

Preliminary interpretation suggested by these facts of thedevelopment of Phoronis™

26 W. H. OALDWELL.

1. A gastraea with slit-like mouth and a pair of lateraldiverticula giving rise to mesoderm was the ancestor ofPhoronis.

2. The rapid growth of ectoderm in the median ventral linenearly succeeded in destroying the continuity of the primitivestreak.

3. The necessity of an early attainment of a terminal positionby the anus caused the ectoderm to grow more rapidly than,the endoblast, and resulted in a division of the mesoderm intoanterior and posterior parts.

4. The nephridia, which might have remained either whollyor in part with the anterior, have attached themselves entirelyto the posterior mesoderm.

Extension of this interpretation to the other Tripleblastiea—1. Phoronis is the first step towards a complete division of

the blastopore. The inducing cause of such division is theelongation, of the body, while the endoblast is still in anembryonic condition.

2. The division of blastopore caused the division of meso-derm.

3. The division of mesoderm results in—i. The masking of the original mode of mesoderm forma-

tion.ii. Metameric segmentation.

IN CAMP, BUENEII RIVER,

QUEENSLAND ; July 27,1884.

BLASTOPOE-E, MESODEBM AND JTETAMEEIC SEGMENTATION. 27

EXPLANATION OF PLATE II,

Illustrating Mr. "W. H. Caldwell's paper on " Blastopore, Meso-derm and Metameric Segmentation."

List of Reference Letter*.

Blastopore, LI. Mouth, «. Anus, a. Primitive streak, p. s. Primitivegroove, p. g. Posterior pit, g. Posterior solid cord. r. Anterior mesodermicdiverticulum, a. d. Posterior mesodemic divertiGniiiin, p. d. Mesoderm, me.Praoral lobe, p. !. Arehenteron, ar. External opening of nepliridium, s.Body cavity (coelom), a. Splancbnopleuric vascular space, v. Nutritive (?)body, x.

All the fignres, both of whole embryos and of sections, were drawn, bymeans of Zeiss's two-prism camera, from permanent preparations in Canadabalsam. The embryos were treated as follows:—Mixture of two relumescorrosive sublimate -f- one vol. glacial acetic acid for 1 second, water 15seconds, alcohol, 50 per cent., 3 minutes, alcohol 70 per cent.

Eigs. 1—7. Zeiss's oe. 2, obj. D. Eigs. 8—18. Zeiss's oc. 2, obj. l-12thkomog. imm. All the embryos beloag to the species of Phoroms, lifing inthe harbour of Naples in dense colonies without sand adhering to their tubes,except that of which Fig. 13 is a section. This is aa Australian PhoroniSjdiscovered in Port JacksoE by Mr. Haswell, of Sydney Uni?ersity. Thesections drawn are taken from complete series. Each sectioE is '005 mm.thick. I have series '0025 mm. thick. These, thongh necessary for observa-tion, purposes, are not so convenient for drawing.

FIGS. 1—4.—Eour embryos, showing mode of closure of the blastopore.Eg. 1. Blastopore, 61., before meeting of lips.Fig. 2. Blastopore fused posteriorly: opes part=mouth, m.; dosed part=

primitive streak aad groove, p. g.Fig. 3. Primitive groove, p.g.\ disappearing praorai lobe, p. I.Kg. i. Primitive groove enlarging into pit, g., posteriorly; prseorai

lobe, p. I.EIGS. 5 aad 6.—Two embryos viewed from the left side. Braeorai lobe,

p. I.; primitive groove, p. g.; mouth, m.; posterior pit, g.Fig. 7.—Older embryo with, complete alimentary canal. Month, m.; anas,

a.; external opening of nephridium, «.FIGS. 8—10.—Three sections in a nearly transverse plane of an embryo in

a stage betweea that of Fig. 1 and that of Fig. 2.

28 W. H. CA.LDWELL.

Fig. 8. In front of mouth. Anterior mesodermie diverticula, a. d.; anteriormesodermic cell, me.; arehenfceron, ar.

Fig. 9. Through the fusing lips of the blastopore. Primitive streak, p. s.;archenteron, ar.

Fig. 10. Through the posterior part of the primitive streak. Primitivegroove, p.g.\ posterior solid cord of cells, r.

FIG. 11.—Median section, longitudinal vertical, through a slightly olderembryo than Figs. 8, 9, and 10. Pneoral lobe, p. 1.; anterior mesoderm, me'.;mouth, m.; archenteron, ar.; posterior pit, g ; posterior solid cord of cells,r; Nutritive (?) body, x; body cavity, c.

FIG. 12.—Embryo slightly older than Fig. ±. Median longitudinal verticalsection. Prsaoral lobe, p. I.; anterior mesoderm, m'.; body cavity, c.; vascu-lar space, v.; middle mesoderm, m".; posterior mesoderm, rd".; archen-teron, ar.; posterior pit, g; Mouth, m.

FIG. 13.—Embryo, Australian species, nearly same stage as Fig. 12.Longitudinal horizontal section. Archenteron, ar.; anterior mesoderm, me';posterior mesodermic diverticula, p. d.; posterior pit, g.

FIG. 14.—Nearly same section as Fig. 13. Vascular space, v.; posteriorpit, g; archenteron, ar.; anterior mesoderm, me".; posterior diverticula,p. d.; posterior mesoderm, me'".

FIG. 15.—Embryo, same stage as Fig. 12. Longitudinal horizontal section.Reopening of posterior cord of cells to form rectum and anus, a.; anteriormesoderm, me'.; archenteron, ar.

FIG. 16.—Embryo, same stage as Fig. 12. Transverse section throughposterior diverticula, p. d.; posterior mesoderm me"'.; arehenteron, ar.

FIGS. 17 and 18.—Embryos about the stage of Fig. 3. Two transversesections. Anterior diverticula, a. d.; body cavity, c.; vascular space, ».;primitive streak, p. s.; primitive groove, p. g.; middle mesoderm, me".; arch-enteron, ar.; nutritive (?) body, x.

FIG. 19.—Six diagrams, illustrating the typical modes of mesoderm forma-tion, e. g.

1. Peripatus, cf. Hertwig's " Coslom-theorie," PI. ii, fig. of insect embryo.2. Amphioxus.3. Nemertine larva of Desor (?), Phoronis (posterior).4. Pristiurus, Phoronis (anterior).5. Lopadorhynchus (Kleinenberg).6. Primitive streak region, many Vertebrates and Phoronis.

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