energetique du developpement de l'œuf
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glticklich, uns auch ein wenig tiber die Ursache aufzukliiren, die die nach einigen Tagen eintretende Azidifikation bewirken mut~. Nach seinen Ver- suchen (I) haben wir diesen Reaktionsumschlag mit anwachsendem CO2-Gehalte der Protoplasten zu erklaren. H. P fe i f fe r (Bremen).
Rapkine, L., Energet ique du Developpement de l'wuf. Chahine, Paris, 1928, pp. 53. Fr. 13.
This valuable monograph is the first that has appeared on the energy- relations of the egg during development from fertilised cell to finished embryo. I t is quite time that some acount was taken of the advances which have been made in our knowledge of this subject, for since the fundamental contributions of TA~GL and his collaborators about 1909 no summary specifi- cally devoted to the question has appeared; although mention of it has been made in certain reviews ((~RAFE, PAETOIINER, NEEDtIAM).
The author begins by a short account of the chemical transformations which proceed during the development of the eggs of the nematode worm, the amphibian, the teleost, tim silkworm, the sea-urchin, and the hen. Although much compressed, the figures given permit of a wide general sur- vey of the most important general events. In discussing the chemical changes in the hen's egg the author devotes special attention to the meta- bolic balance-sheet which the recent work of H. A. MURRAY and J. N]~EDItAM have established for this closed system. In discussing the respiratory changes he draws special attention to the heat-absorption found by BoH & HASSEL- ~AC~ during the first week of avian development, to the enigmatic Calorific Quotients of MEY~RI~0~ 1 on echinoderm embryos and to his own interesting measurements of carbon dioxide production and oxygen-consumption of sea- urchin eggs.
The more original part of the work, however, is to be found in the discussion of the Developmental Efficiency or "rendement energetique"; a concept of considerable biological importance. After discussing the manner of calculating it known through the work of TERROINE WURlgSER, which leads to efficiencies of roughly 60~ even when allowance has been made for energy of maintenance or basal metabolism, the author suggests that a new way of calculating the efficiency might be desirable.
He points out that the usual equation: * U j
E - - u - - (uR + us)
where E is tim efficiency, U ~ the energy stored in the embryo at the end of development, U the energy stored in the embryo at the end of develop- ment, U the energy stored in the raw materials at zero hour, UR the energy present at the end of development in the unused raw materials, and UE the energy in the solids burned for basal metabolism, will do very well as long as complete combustions to CO.) and HeO are alone considered, but not when more complicated processes are involved. The developing organism may have at its disposal other sources of energy, for endothermic reactions are known to occur in vivo, reactions which raise the chemical potential of their products above that of the reactants. Some confusion arises here owing to the fact that certain workers use the term "energy-sources" to apply to the solids burned to give the heat lost from the egg, while others (including the present author) only use it to apply to those reactions,
whatever they may be, which give the energy of organisation, i .e. that fraction of energy which is employed in the actual work of formation. BOltR & HASSELBALCtt showed finally that of the former lot of energy not more than 4 ~ can participate in the work of formation, but when we take into account the energy not furnished by complete combustions, it is legitimate to suppose that a larger proportion of the total energy turnover may be used for work of formation. This is the essence of the author's thesis.
Whatever this fraction of energy is we cannot expect to find it by bomb calorimetry for the organisation of the tissues is destroyed in that process. The author focusses attention on coupled spontaneous endothermic reactions and considers they explain the peculiar phenomena referred to above (BoRR & HASSELBALCH, ~r I:~APKINE) and perhaps also the synthesis of fatty substances which takes place in the eggs of some fishes and gastropods.
Applying these ideas to the efficiency formula, the author suggests that the numerator (U' i. e. cals. in unit weight of the finished embryo) should be replaced by U ' - - (minus) the energy contained in an exactly equivalent weight of original raw material. This would represent the ele- vation of calorific value which has gone on during development.
U ~ - - x
U - - (UR + U~) Evidently this will give an efficiency of a very low order. And it
will not be comparable with the "rendement energetique brut" or the "rendement energetique reel" of other workers for what they measure is the relation between the energy stored in the embryo or transformed into its tissues on the one hand and the energy absorbed by the embryo from its raw materials on the other hand, either allowing for the basal metabolism or not. RAPKiNE'S efficiency coefficient, on the contrary, measures the relation between the energy furnished to the embryo from coupled reactions etc. (i. e. energy which would have been dissipated as heat if the endothermic pro- cesses had not caught and held it within the egg) on the one hand, and the energy absorbed by the embryo from the raw materials on the other hand, allowing for the basal metabolic requirements. I t thus has to do, not with energy-storage as a whole, but with the storage of energy from a particular source, i. e. coupled reactions with one endothermic component. Put in another way, the "rendement energetique brut'" and the "rendement energe- tique reel" express the amount of total storage that can take place during a given amount of absorption, but I:~APKINE'S efficiency coefficient would express the amount of storage of energy that would otherwise have been lost as heat, that can take place during a given amount of absorption. Thus during the absorption of 100 cals. of energy, 66 will be stored and 33 burned, but only 9, say, out of the 66 will have been saved from the burning by the endothermic processes.
Now at first sight I~APKINE'S efficiency coefficient would seem to be a minus quantity, for in MURRAY'S work for instance, the calorific value of 1 gm. of dry unincubated yolk and white (mixed) was 6"94 cals. and that: of 1 gin. of dry finished embryo was 6"2 cals. No increase in specific energy-content would seem, then, to have taken place. However, the finished embryo is, as RAP~mE points out, not comparable with the unincubated yolk and white, for a notable proportion of the fat of the former disappears by combustion and some of it is left behind in the spare yolk at hatching. The following calculation is therefore required to give the energy-value of an amount of yolk and white roughly comparable to the finished embryo:
The finished embryo of the chick weighs 6"00 gins. dry weight has in it 37"5 Cals.
i.e. 6"2 Cals. per gin. dry weight. The egg at the beginning of development has in it
12"45 gins. dry weight 86'85 Cals.
i.e. 6"94 Cals. per gin. dry weight. Cals. ~
Subtract . . . . . . . . . . . 86"85 100 for COMBUSTION
2'5 gms. of fat are used, which at 9"3 Cals. per gin. is . .
for YOLK UNUSED at the end about 4'75 gins. dry weight of which 400/0 is fat and 500/0 protein making a total of
29"20 52"45 86.85 52'45 34"40
An amount of yolk and white, therefore, equivalent to the finished embryo has 34"40 Cals. as against its 37'5 Cals.
But from this 34"4 Cals. must be subtracted a correction for the skeletal system of the finished chick which will contain very little energy. The 37,5 Cals. is the heat contained, not in 6"0 gins. dry wt. but in 4"5 gins. dry wt. (for the bones weigh about 1"5 gins.), i. e. 75 ~ of 34"4 Cals. will give the energy of an amount of yolk and white equivalent to the finished embryo. "This is 25"7 Cals. 37"5 - - 25"7 Cals. ~ 11"8 Cals. is the energy stored in the embryo by the endothermic components of coupled reactions, i. e. increase of chemical potential. Applying this to the efficiency formula, we get
37"5 - - 25'7 86.95 - - (26"4 ~- 17 (say)) X 100 - - 27'15 ~
This calculation is only of illustrative significance for the data are taken from various different sets of material, but it should show, nevertheless, that the new efficiency coefficient will always work out at a 10w level, under thirty per cent. RAPKI~E suggests that it may be possible to discover what the coupled reactions in question are, by placing various hydrogen donators in the presence of embryonic tissues at different stages of deve- lopment and following electrometrically their dehydrogenation.
The monograph is as a whole very suggestive. It marks the end of an epoch. From the days of VAZENCmNSES & FREMY or of PREVOST & MORIN up to the present time, chemical embryologists have thought exclusively in terms of simple storage and simple complete combustions to carbon dioxide and water. In the light of these straightforward ideas great advances have been made, but it is now time to realise that the events in the developing egg are probably a good deal more complicated, and involve other processes as well. The merit of RAPKmE'S book is that he points this out, and looking forwards as well as backwards, summarises the results of the past and indi- cates the most hopeful paths for the work of the future.
JosEe~ NEEDHA~ (Cambridge).