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-Pig, 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. 
