4I 8 COMPARATIVE ZOOLOGY 



tain the nervous cord : while that of the invertebrate 

 egg forms only one such tubular division. The features 

 which determine the branch to which an animal belongs 

 are first developed, then the characters revealing its 

 class. 



There are differences also in grade of development as 

 well as type. For a time there is no essential difference 

 between a fish and a mammal; they have similar ner- 

 vous, circulatory, and digestive systems. There are 

 many such cases, in which the embryo of an animal 

 represents the permanent adult condition of some lower 

 form. In other words, the higher species, in the course 

 of their development, offer likenesses, or analogies, to 

 finished lower species. The human germ at first re- 

 sembles that of all other metazoa in that it is a single 

 cell. In the course of its development, the appearance 

 of a medullary furrow excludes it at once from all inver- 

 tebrates. It afterward has, for a time, structures found 

 as permanent organs in the lower classes and orders of 

 vertebrates. For a time, indeed, the human embryo so 

 closely resembles that of the lower forms as to be indis- 

 tinguishable from them ; but certain structures belong- 

 ing to those forms are kept long after the embryo is 

 clearly human. For instance, the embryos of birds and 

 mammals at an early stage have gill slits, like fishes. 

 Not all the members of a group reach the same degree 

 of perfection, some remaining in what corresponds to 

 the immature stages of the higher animals. Such may 

 be called permanently embryonic forms. 



Sometimes an embryo develops an organ in a rudimen- 

 tary condition, which is lost or useless in the adult. 

 Thus, the Greenland whale, when grown up, has not a 

 tooth in its head, while in the embryo life it has teeth in 

 both jaws ; unborn calves have canines and upper in- 

 cisors ; and the female dugong has tusks which never 



