108 ADAM SEDGWICK. 



segmented Triploblastica are in all cases derived from a primi- 

 tive single mouth ; that this primitive mouth is represented in 

 the embryo by the blastopore which should, if the phylogenetic 

 development were repeated, give rise directly to the mouth and 

 anus. I explained the fact that the blastopore so rarely does 

 give rise to the mouth and anus by supposing that it became 

 specialised as a larval ^ structure. My view is that in those 

 animals in which it does not give rise to the mouth and anus, 

 it functioned as the larval mouth while the animal was devel- 

 oping, and persisted until parts of the embryo were developed 

 between it and the position of the mouth and anus of the 

 adult, which parts had arisen in the phylogenetic history in the 

 adult after the primitive mouth had completely divided into 

 the mouth and anus. These parts never had been traversed by 

 the original slit-like mouth, because they had appeared at a 

 stage in evolution subsequent to the stage in which the mouth 

 and anus were one. It cannot therefore be a matter of 

 surprise if the blastopore does not elongate and bisect these 

 later structures, which never had in the history of the animal 

 been perforated by the blastopore. It is very difficult for me 

 to express my meaning in clear language, and I am driven to 

 take an instance to illustrate it. According to my view the 

 cerebral hemispheres have appeared at a stage in the evolution 

 of the Vertebrata long after the primitive mouth has become 

 separated into the mouth and anus. The blastopore (primitive 

 mouth), however, which has in some ancestral Vertebrate 

 functioned for a considerable time in the larva as the only 

 opening into the alimentary canal, persists and does not elongate 

 to give rise to the mouth and anus which are not formed until 

 after the cerebral hemispheres have appeared. It is now no 

 longer possible, nor would it be advantageous if it could, that 

 the specialised blastopore should elongate and give rise to the 

 mouth and anus, the middle part closing up. The cerebral 

 hemispheres have appeared, and they have never in the phylo- 



' The larval stage, for which the mouth was specialised, has in the Verte- 

 brata, as in many other animals, vanished ; it has probably been included in 

 the embryonic period, and is rapidly hurried over. 



