THE HEAD PROBLEM 



617 



connexion between the premandibular cavity and the hypophysis dis- 

 covered by Goodrich ('17) in Torpedo. The fact is that in the develop- 

 ment of the anterior cavities, as in many other respects, Squalus and 

 Galeus present more primitive characteristics than do most elasmobranch 

 embryos. While petromyzon, as Goodrich says, does not have anterior 



ANT. ENDODERMIC DIVERTICULUM 



ANT ENCXDDERM. 

 DIVERTICULUM 



A. 



.BRAIN 



ENTERON' 



a: b.' c' 



Fig. 510. — Diagrams of frontal and cross sections of Amphioxus (cephalochordate), 

 Amia (ganoid), and Squalus (elasmobranch) embryos, showing the exact homology of 

 the anterior endodermic diverticula of Amphioxus, the adhesive organs of Amia, and 

 the anterior somites of Squalus. A, A', A^, A^ sections of amphioxus; B, B' sections of 

 Amia; C, C sections of Squalus. 



cavities, there are mesenchyma cells anterior to the premandibular somite 

 which may represent the anterior cavities. 



The assertion that the anterior cavities "have not been found at all 

 in other gnathostomes" may be questioned, since Reighard ('02) has 

 shown that the adhesive organs of amia, a ganoid fish, resemble the 

 anterior cavities both in development and in their relations to the preoral 

 gut and to the premandibular somite, and must therefore be considered 



