Presidential Address. 



19 



Similarly in regard to the nervous system, all the probabilities seem to point 

 to its having become evolved out of a subepithelial nerve plexus of the type 

 which still persists in Ccelenterates, Echinoderms, etc. We may take it that 

 this nerve plexus was not absolutely restricted to the ectoderm, it probably 

 showed more or less extension under that part of the bounding epithelium 

 which became invaginated to form the endoderm. 



Fig. 6. 



Diagrammatic transverse sections through primitive Metazoan organisms 

 possessing a protostoma or primitive mouth (p.s. ). 



In each a concentration of nervous system (e.n.s.) is shown on each side 

 of the protostoma. 



In the lower figure, which represents a more advanced stage in Evolution, 

 coelenteric pouches are becoming marked off by the down growth of a chink 

 or involution of the endoderm between the points a and b. 



Now it is well known that in the ontogeny of various Vertebrates there 

 exists in early stages of development continuity between the cell layers along 

 the mid-dorsal line — a very puzzling phenomenon — the only plausible explana- 

 tion of which so far suggested is, that this line of continuity is a vestige of a 

 once present dorsally situated slit-like mouth or protostoma. 



