STRUCTUKE OF DOMESTICATED ANIMALS. 301 



Figs. 1, 2 and 5, 3 and 4, 6 and 7 show that the eggs of differ- 

 ent 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. 



Ovarian egg of rabbit. Copied from Bls- 

 ohoS 's embryology of the rabbit. Magni- 

 fied 125 diameters. 



Fig. 6. 



Ovarian egg of rabbit, freed of the cells which 

 surround the zona pellucida in fig. 5. Copied 

 from Bischoff. Magnified 125 times. The ger- 

 viinative vesicle fehines through the yolk as a 

 light spot. 



Fig. 7. 



The same ovarian egg of the rabbit as in 

 fig. 6, opened with a needle. The yolk, 

 with the germinative vesicle and dot are 

 flowing out. Copied from Bischoff. Mag- 

 nified 125 times. 



Fig. 8. 



Ovarian egg of a human female, cut open. The 

 yolk has escaped whole, and in it the germi- 

 native vesicle and germinative dot are seen as 

 a lighter spot. Copied from Bischoff. Mag- 

 nified 100 times. The resemblance to the eggs 

 of the rabbit and dog represented in figs. 4 

 and 7, is very striking. 



The formation of a germ in the Q;^g begins by a very pecu- 

 liar process, called " segmentation." It is unquestionably a man- 

 ifestation of the internal life of the Qgg, — for an e^g must be 

 considered as a living body. " Segmentation " con&ists in this : 

 Supposing we have here the egg of a dog, 

 copied from Bischoff (fig. 9) : 

 the egg divides itself spon- 

 taneously into two halves (fig. 

 10) , which are entirely inde- 

 pendent of one another, and 

 Fig- 9- only retained together by the •p\g. lo. 



common envelope of the yolk. After that, each half divides 



