HOMOLOGY. 



13 



the cranial roof-bones, is only arrested in its aim to form a 

 mouth, or " deutostome," at the vertex, by failing to over- 

 come the resistance of the superincumbent epithelial layer — • 

 such resistance being encouraged by the processes now on 

 foot to estabHsh an external communication, elsewhere, with 

 the fore part of the alimentary canal. 



Embryological researches, subsequent to those given in the 

 'Memoir,' communicated, September 5th, 1881, to the 



British Association at the York meeting, have concurred in 

 demonstrating the extension of the primary alimentary canal 

 " cephalad" and "neurad," prior to the haemal opening into 

 the wide branchial space or sac. I take the two illustrations, 

 figs. 5 and 6, from the valuable paper " Du Developpement 

 des Ventes et Arcs Branchiaux chez I'Embryon," by Professor 



The extension of the fore part of the alimentary canal in 

 * ' Journal d'Aiiatomie et de Physiologie,' 1883, p. 38, pis. v.-viii. 



Pig. 5. 



Cadiat*- 



