636 



COMPARATIVE ANATOMY 



nation which forms the mouth in annelids to the ectodermal hypophysis in 

 cyclostomes. 



Minot's hypothesis involves three assumptions: 



1. The preservation of the ectodermal stomodeum of annelids as 

 the naso-hypophysial invagination of vertebrates. 



2. The formation of the vertebrate brain from several of the post- 

 oral ganglia of the ventral chain. 



3. The conversion of the visual apparatus and supra-esophageal 

 ganglia, which are prevented from fusing in the median line by the 

 enlargement of the vertebrate brain, into the vertebrate retina, the 

 esophageal commissures persisting as optic nerves. In Minot's opinion, 

 this hypothesis obviates those difficulties of the Dohrn-Semper annelid 

 theory which at present are the most serious. He explains the absence 

 of eyes in amphioxus as the result of degeneration. Concerning the 

 failure of embryology to confirm his speculations Minot is silent. 



MOUTH STOMODAEUM CARDIOPORECBLASTOPORE; 

 J 



PREORALLOBE 



A. Annelid 



ANUS 

 NEURENTERIC 

 DORSAL NEURAL TUBE CANAL 



B.ACRANIATE. 

 ARCHEJ4CEPHAL0N 



NEURAL TUBE 



NEURENTERIC CANALCBLASTOPORE) 



NEUROPORE 



C. Craniate 

 Fig. 521. — Diagrams of A, Annelid, B, Acraniote, C, Craniote. According to Dels- 

 man the ectodermal stomodoeum of annelids is converted in vertebrates into the neural 

 tube. Amphioxus is a transitional form between annelids and vertebrates. The 

 blastopore of annelids becomes the neurenteric canal of chordate embryos. (Redrawn 

 after Delsman slightly modified in lettering.) 



DELSMAN'S THEORY 



A modified annelid hypothesis has been advanced by Delsman (19 13), 

 not, however, as an attempt to render the views of Dohrn and Semper 

 acceptable, but as an entirely new theory which grew out of embryological 

 researches on invertebrates. The gist of Delsman's theory is that the 

 neural tube of vertebrates was originally a part of the alimentary canal 

 of invertebrates and that it corresponds to the ectodermal stomodeum 

 of invertebrates such as the annelids. 



Kowalewsky (1877) was the first to suggest that the neural tube may 

 originally have functioned as a part of the digestive system. Sedgwick 



