6i8 



COMPARATIVE ANATOMY 



to be homologous with them. There is evidence also that they are 

 possibly homologous with the pit-organs of vipers. 



The facts of development do not bear out the assertion that the 

 anterior cavities are segregated portions of the premandibular cavities. 

 It would be equally misleading to state that the premandibular cavity 

 is a segregated portion of the mandibular cavity. The three structures 



ANLAGE 



ENDODERMIC 



DIVERTICULUM 



SOMITES' 

 A. AMPHIOXUS 



/(SOMITES 



C. SQUALUS ^YOUK 



Fig. 511. — Parasagittal sections showing the similarity of the relations of the 

 anterior endodermic diverticula of Amphioxus (A) to those of the adhesive organ of 

 Amia (B) and of the anterior somites of Squalus (C). Each of these "cavities" arises 

 immediately in front of the first permanent myotome (somite i). 



are, in fact, serially homologous — at least in part — and have equal seg- 

 mental value. This conclusion is supported by the resemblance of the 

 anterior cavities to the anterior endodermic diverticula of amphioxus. 

 Like the latter they are the anteriormost cavities of the body and develop 

 immediately anterior to the first permanent myotomes. The resemblance 

 of the adhesive organs of amia to the anterior endodermic diverticula 

 of amphioxus is even more striking. Both arise as paired endodermic 

 diverticula of the anterior end of the preoral intestine. Both open by 

 pores to the exterior, the left cavity in amphioxus forming the preoral pit. 



