88 MAN AXD THE LOWER ANIMALS. n 



There is not much apparent resemblance be- 

 tween a barn-door Fowl and the Dog who protects 

 the farm-yard. Nevertheless the student of de- 

 velopment finds, not only that the chick com- 

 mences 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 

 undergoes division, or segmentation as it is often 

 called: the ultimate products of that segmenta- 

 tion 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 dif- 

 ferences between them are inconsiderable, while, 

 in their subsequent course they diverge more and 



