56 Growth in length of the Vertebrate Embryo 



perhaps also be regarded as a necessary culmination of proto- 

 genesis). If so these consequences of embryonic development 

 must have had their effect upon evolution. For instance, one 

 of the first effects of deuterogenesis must be in many cases a 

 tendency to close the blastopore. 



Now clearly in evolution to close the only entrance to the 

 digestive tract would be fatal. Therefore there must always have 

 been and must always be the counteracting circumstances of the 

 "necessity to live" opposing the geometrical tendency, just as 

 there always are and always have been the counteracting 

 influences of the force of gravity which would bring us to the 

 earth, and the necessities of living that keep us erect. The 

 continual warring of these tendencies at each repetition of the 

 life cycle, the one to close, the other to keep open the gastrula 

 mouth, may well have given the impetus in evolution which has 

 led to the very varying fates of the blastopore in the different 

 groups of animals, and the consequent relation of main axis to 

 the plane of the blastopore. 



The two phenomena, the difference in the fate of the blasto- 

 pore, and the difference in the fate of the deuterogenetic centre, 

 are of fundamental importance in our attempt to understand the 

 relationship of the chief phyla of the animal kingdom. Let us 

 take a very brief survey of what we know of the actual fate of 

 the blastopore in individual species. In Coelenterates the fate is 

 simple. The blastopore remains open, acting as chief inhalent 

 and exhalent aperture. The deuterogenetic centre, if there ever 

 is one, dies out wholly and at once. In Echinodermata the blasto- 

 pore may be said to remain, but as anus only. A new opening 

 is formed, which is the mouth. The deuterogenetic centre probably 

 dies out wholly and at once. In the Chordata the blastopore re- 

 mains in part as anus or else the anus opens where the blastopore 

 has closed. We may regard this as a re-opening of a part the 

 ventral part of the blastopore. To be accurate it is the more 

 ventral part of the blastopore which is anus, the dorsal part, after 

 becoming enclosed by the neural folds forms the neurenteric canal, 

 which ultimately closes. The mouth is never formed from any 

 part of the blastopore; it is always a new opening. The whole 

 deuterogenetic area continues active for a short time, then the 



