ZOOGENESIS 



these major groups in the larval as well as in the 

 adult stages. 



It is an interesting fact that the embryological 

 development of the cephalochordates, balanoglossids, 

 cephalodiscids, tunicates and vertebrates indicates a 

 close relationship between these five groups, which 

 are commonly referred to as the chordates and the 

 protochordates. 



This indicates that the increased structural com- 

 plexities of these groups do not permit of much 

 deviation from a general mean. It cannot be inter- 

 preted as indicating that these five groups were ever 

 more closely related than they are now, or that any 

 one of them was derived through or from any of the 

 others. Each represents a separate and distinct path 

 from the gastrula, though these paths are but little 

 divergent. 



According to this interpretation the various phyla 

 of bilaterally symmetrical animals are in effect recom- 

 binations of features which are inherent in animals 

 taken as a whole, or in other words recrystallizations 

 of the fundamental features of animal organization, 

 which occur at every focal point where an animal type 

 capable of existence may be formed from the elements 

 available in the general animal complex. 



No appreciable time element is necessarily involved 

 in such a process of recombination or recrystalliza- 

 tion of fundamental animal features. Therefore at the 

 very first appearance of life the animal world, so far as 

 the phyla or major groups of animals are concerned, 

 probably was quite the same as it is today. 



[199] 



