88 MAN AND THE LOWER ANIMALS n 



born, and then, by still slower and less perceptible 

 steps, passes into the adult Dog. 



There is not much apparent resemblance 

 between a barn-door Fowl and the Dog who 

 protects the farm-yard. Nevertheless the student 

 of development finds, not only that the chick 

 commences its existence as an egg, primarily 

 identical, in all essential respects, with that of 

 the Dog, but that the yelk of this egg undergoes 

 division that the primitive groove arises, and 

 that the contiguous parts of the germ are 

 fashioned, by precisely similar methods, into a 

 young chick, which, at one stage of its existence, 

 is so like the nascent Dog, that ordinary inspection 

 would hardly distinguish the two. 



The history of the development of any other 

 vertebrate animal, Lizard, Snake, Frog, or Fish, tells 

 the same story. There is always, to begin with, an 

 egg having the same essential structure as that 

 of the Dog : the yelk of that egg always under- 

 goes 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 



