650 STRUCTURE AND GROWTH OF DOMESTICATED ANIMALS. 
Figures 154, 155, 156, 157, 158, 159 and 160 show that the eggs 
of different mammalia, such as rabbits and dogs, resemble one 
another as much as the eggs of different species of birds belonging 
to different orders of this class. 
The formation of a germ in the egg begins by a very peculiar 
process, called “segmentation.” It is unquestionably a mani- 
festation of the internal life of the egg,—for an egg must be 
Fig. 154, Fig. 155. 
Ovarian egg of dog. Copied from Bischoff’sem- Another ovarian egg of dog, from a female 
bryology of the dog. Magnified 100 diameters, egyes heat, ri ea from Bischoff. Magni- 
ed 100 
Fig. 156 Fig. 157. 
Ovarian egg of dog, freed of the cells The same ovarian egg as that represented in fig. 
which surround the zona pellucida in 156, cut a a uhhh sharp needle. The rice 
figs. 154 and 155. Copied from Bischoff. escaping is yolk. with the transparent germ me. 
Magnified 100 diameters, tive vesicle, in which the gorami dotis = . 
Copied from Bischoff. Magnified 100 tim 
eate as a living body. Segmentation consists in this. 
Supposing we have here the egg of a dog, copied from Bischoff 
(fs. 162) : the egg divides itself spontaneously into two halves (fig. 
163). which are entirely independent of one another, and only re- 
tained together by the common envelope of the yolk. After that, 
each half divides itself into two halves again, so ‘that the yolk 
