Mammalian Eggs 157 



The small egg of the mammal divides into a number 

 of cells, which form a hollow sphere ; on the upper sur- 

 face of this the development of organs begins with the 

 formation of a depression which indicates the future 

 middle line of the animal, and is, in fact, the beginning 

 of the nervous system. Under this is formed a straight 

 rod of gelatinous material, the foundation of the verte- 

 bral column, and the body of the embryo is gradually 

 pinched off from the surface of the hollow sphere. 

 After tracing the details of this process, Huxley pro- 

 ceeded as follows : 



"The history of the development of auy other vertebrate 

 animal, lizard, snake, frog or fish, tells the same story. There 

 is always, to begin with, an eg^, having the same essential 

 structure as that of the dog ; the yelk of that egg always 

 undergoes division, or segmentation, as it is often called ; the 

 ultimate products of that segmentation constitute the building 

 materials for the body of the young animal; and this is built 

 up round a primitive groove, in the floor of which a notochord 

 is developed. Furthermore, there is a period in which the 

 young of all these animals resemble one another, not merely in 

 outward form, but in all essentials of structure, so closely, that 

 the differences between them are inconsiderable, while in their 

 subsequent course they diverge more and more widely from 

 one another. And it is a general law, that, the more closely 

 any animals resemble one another in adult structure, the lon- 

 ger and the more intimately do their embryos resemble one 

 another ; so that, for example, the embryos of a snake and of a 

 lizard remain like one another longer than do those of a snake 

 and of a bird ; and the embryos of a dog and of a cat remain like 

 one another for a far longer period than do those of a dog and 

 a bird ; or of a dog and an opossum ; or even than those of a 

 dog and a monkey." 



This general rule, that the longer the paths of em- 

 bryonic development of two animals keep identical the 



