TfJ ZOOGENESIS l"^^^ 



animal with half the bulk of the parent. In all other creatures 

 the original germ cell divides into two as in the case of the proto- 

 zoans, but the two derivatives of the original germ cell remain 

 connected. 



So the difference between the protozoans and the other animals 

 is chiefly and most obviously a difference in the behavior of the 

 single cell which lies at the base of all animal structure. 



The essential and most obvious difference between the sponges 

 and the coelenterates is that in the sponges the cells as they divide 

 adhere in a more or less irregular mass in which many of them 

 retain to a large extent their individuality, while in the coelenter- 

 ates the cell division takes place geometrically and results in the 

 formation of a radially symmetrical body in which all of the cells 

 • — or practically all of them — have entirely lost their individuality 

 through incorporation into various organs and structures. 



Thus the difference between the sponges and the coelenterates 

 has its inception in the very early embryonic stages. But it 

 would seem that the sponges and the coelenterates are more 

 closely related to each other than either group is to the proto- 

 zoans in which there is no adhesion of cells — or at least no adhe- 

 sion which impairs the complete individuality of the cells. 



The curious assemblage of creatures with which we are at 

 present concerned seems clearly to be less like the sponges than 

 like the coelenterates. But in their embryonic stages they depart 

 very early from the regular geometrical line which leads to the 

 adult coelenterate. Their relation to the coelenterates is of 

 approximately the same nature as the relationship between the 

 sponges and the coelenterates, or as the relationship between the 

 sponges and the coelenterates taken together and the protozoans. 



Any regular geometrical line of development would naturally 

 be subject to every possible deviation and departure from the 

 normal. Such of these deviating lines of development as would 

 result in producing an adult animal — no matter how bizarre — 

 which has an internal chemical and physical balance enabling it 

 successfully to carry on the necessary vital processes, and at the 

 same time was fortunate enough to find some niche in which it 

 could maintain itself, could very easily produce an apparently 

 wholly new type of animal. 



Such new types of animals, coming into existence as the result 

 of deviations from the regular developmental line originating at 



^45] 



