58 



THE MUSEUM. 



coon, and this would be gorged with 

 the food-yolk obtained by the annex- 

 ation of its fellows. 



The original cocoon by the solidifi- 

 cation of its outer walls, its center re- 

 maining semi-fluid, might thus become 

 an egg-case for a single egg. This 

 would be realized in fact, if the eggs to 

 be consumed, instead of being deposit- 

 ed as separate entities within the co- 

 coon, were joined to or devoured by 

 that single one which was destined to 

 develop while in the ovary. The well- 

 known instances of Hydra and Tubul- 

 aria, with their ovarian "cannibalism" 

 are obviously illustrative. 



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 becomes 

 an egg-case for a single egg. 



Though such an egg containes a 

 large amount of food-yolk in many in- 

 stances, this cannot be retained during 

 cleavage and the subsequent processes 

 within the cells of the developing or- 

 ganism, 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 re- 

 linquish 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 indiffer- 

 ence whether it be added to or de- 

 voured by the ovarian egg, or whether 

 it be first obtained by the swallowing 

 of other eggs within the cocoon or egg- 



case. For the latter case ultimately 

 resolves itself in a mere modification of 

 the latter. To put it in another way, 

 the devouring of the yolk-mass has in 

 both cases to be undertaken by the 

 developing organism before it can be 

 said to have obtained real possession 

 of the yolk. 



ft is thus that the growth of a blas- 

 toderm, i. e. , of a flattened gastrula 

 over a yolk-sac, is exactly comparable 

 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 not got but a little dis- 

 tance along the path of evolution of 

 animal eggs, and others interesting re- 

 main for examination. On another 

 occasion the study of the purse of a 

 dog-fish or skate may engage our at- 

 tention. From this we may pass to 

 the consideration of the egg of a mam- 

 mal, and finally, this should lead to 

 the elucidation of some of the princi-- 

 pies underlying mammalian develop- 

 ment. 



Food-yolk will here again be found 

 to form an integral part of the pro- 

 gramme, and we may perhaps come to 

 realize with what nicety nature be- 

 stows it on animal eggs. And if we 

 glean nothing else, we may at least 

 learn to realize that the reign of law 

 pervades animal development from its 

 commencement to its close. 



Notes on the Helix of the West In- 

 dian Islands, Including Cen- 

 tral and South America. 



During the last few years the study 

 of the family Hclicida- has been taken 

 up quite generally by conchologists 

 tlie world over, and we feel that no 

 excuses are necessary, in giving to our 

 readers a few facts concerning the 

 Helix of the region named above. All 

 of the area south of the U. S. includ- 

 ing the West India Islands, and South 

 America, with its adjoining Island 

 groups, are included in what is termed, 

 the West India fauna. We know no 



