230 EVOLUTION AND ANIMAL LIFE 



differentiated cells, and then a blastoderm consisting of a single 

 layer of similar undifferentiated cells. But soon in the course 



*> 



of development the embryos begin to differ, and as the young 

 animals get further and further along in the course of their 

 development, they become more and more different until 

 each finally reaches its fully developed mature form, showing 

 all the great structural differences between the starfish and 

 the dove, the beetle and the horse. That is, all animals begin 

 development apparently alike, but gradually diverge from each 

 other during the course of development. 



There are some extremely interesting and significant things 

 about this divergence to which attention should be given. 

 While all animals are apparently alike structurally J at the 

 beginning of development, so far as we can see, they do not 

 all differ noticeably at the time of the first divergence in de- 

 velopment. The first divergence in development is to be 

 noted between two kinds of animals which belong to different 

 great groups or classes. But two animals of different kinds, 

 both belonging to some one great group, do not show differences 

 until later in their development. This can best be understood 

 by an example. All the butterflies and beetles and grass- 

 hoppers and flies belong to the great group or class of animals 

 called Insecta, or insects. There are many different kinds of 

 insects, and these kinds can be arranged in subordinate groups 

 (orders), such as the Diptera, or flies, the Lepidoptera, or butter- 

 flies and moths, and so on. But all have certain structural 

 characteristics in common, so that they are comprised in one 

 great class the Insecta. Another great group of animals is 

 known as the Vertebrata, or backboned animals. The class 



1 We can say that they are alike structurally, only when we consider 

 the cell as the unit of animal structure. But, that the egg cells of different 

 animals differ in their fine or ultimate structure, seems certain. For each 

 one of these egg cells is destined to become some one kind of animal, and 

 no other; each is, indeed, an individual in simplest, least developed con- 

 dition of some one kind of animal, and we must believe that difference in 

 kind of animals depends upon difference in structure in the egg itself. 

 Indeed Wilson, the foremost American student of egg structure, believes 

 himself able to perceive in many eggs a structural differentiation within 

 the egg protoplasm itself, corresponding, in some measure, with the struc- 

 tural differentiation of the embryonic animal as revealed in early develop- 

 mental stages. 



