278 THE OVARIAN EGG. 



regular segmentation changes its character, and 

 at a certain period, in a more or less advanced 

 stage of the segmentation, according to the species, 

 each portion of the yolk assumes an individuality 

 of its own, and, instead of uniting again with the 

 rest, begins to subdivide for itself. In our Natica 

 heros, for instance, the common large gray Sea- 

 Snail of our coast, this change takes place when 

 the yolk has subdivided into eight or sixteen 

 parts. At that time each portion begins a life of 

 its own, not reuniting with its twin portions ; so 

 that in the end, instead of a single embryo grow- 

 ing out of this yolk, we have from eight to six- 

 teen embryos arising from a single yolk, each 

 one of which undergoes a series of develop- 

 ments similar in all respects to that by which 

 a single embryo is formed from each egg in 

 other animals. We have other Naticas in which 

 the normal number is twelve; others, again, in 

 which no less than thirty-two individuals arise 

 from one yolk. But this process of segmenta 

 tion, though in these animals it leads to such a 

 multiplication of individuals, is exactly the same 

 as that discovered by K. E. von Baer in the egg 

 of the Frog, and described and figured by Pro- 

 fessor Bischoff in the egg of the Rabbit, the Dog, 

 the Guinea-rig, and the Deer ; while other em- 

 bryologists have traced the same process in Birds, 

 Reptiles, and Fishes, as well as in a variety of 

 Articulates, Mollusks, and Radiates. 



