1899] PRINCIPLES OF ANIMAL DEVELOPMEXT 137 



When this transference to the ovary of the process of devouring 

 took place, is of course a moot question, but it must have happened 

 at some time or other in many cases. The originally large number of 

 eggs in the cocoon then becomes reduced to a single large one by the 

 conversion of the rest into yolk-material in the ovary itself. And 

 thus, having at first served a rather different purpose, the cocoon be- 

 comes an egg-case for a single egg. 



Though such an egg contains a large amount of food-yolk in many 

 instances, this cannot be retained during cleavage and the subsequent 

 processes within the cells of the developing organism, and these act as 

 if they were not connected with such an enormous mass of food. 

 The egg now, as in a skate, segments in a disc-like fashion, or is 

 " meroblastic." The object of the developing organism now seems to 

 be to regain possession of the food-mass, which it had been obliged to 

 relinquish during cleavage. This it does by forming once more a 

 gastrula. The latter is flattened out on the top of the mass of yolk, 

 and in order once more to annex this yolk, the gastrula must grow 

 round and enclose it. 



So that, if the mass of yolk be large, it appears to be a matter of 

 indifference whether it be added to or devoured by the ovarian egg, or 

 whether it be first obtained by the swallowing of other eggs within the 

 cocoon or egg-case. For the former case ultimately resolves itself in 

 a mere modification of the latter. To put it in another way, the de- 

 vouring of the yolk-mass lias in both cases to be undertaken by the 

 developing organism before it can be said to have obtained real posses- 

 sion of the yolk. 



As I have elsewhere written, 1 it is thus that the growth of a blasto- 

 derm, i.e. of a flattened gastrula over a yolk-sac, is exactly compar- 

 able to the devouring of yolk-masses by a whelk-gastrula. The end is 

 the same, and probably the beginning was the same in both. 



We have now got but a little distance along the path of evolution 

 of animal eggs, and other interesting problems remain for examination. 

 On another occasion the study of the purse of a dog-fish or skate may 

 engage our attention. From this we may pass to the consideration of 

 the egg of a mammal, and finally, this should lead to the elucidation of 

 some of the principles underlying mammalian development. Food-yolk 

 will here again be found to form an integral part of the programme, 

 and we may perhaps come to realise with what nicety Nature bestows 

 it on animal eggs. And if we glean nothing else, we may at least 

 learn to realise that the reign of law pervades animal development from 

 its commencement to its close. 



1 "The Span of Gestation," Jena, Gustav Fischer, 1897. 

 University, Edinburgh. 



