474 "Journal of Comparative Neurology and Psychology. 



themselves into the walls of the anterior head cavities. The mesen- 

 chymatous cells beneath the front end of the brain are in over- 

 whelming proportion of ectodermal (neural crest) origin. 



In Dohm's beantiful plates illustrating his articles on the man- 

 dibular and premandibular cavities, this terminal neural crest is 

 clearly shown. See especially PI. 9, Figs. 7-12. Dohrn does not 

 distinguish between premandibular mesoderm and the mesectoderm 

 of neural crest origin. The latter he calls the anterior part of the 

 premandibular mesoderm (Prsem. 1) after the ''infundibulum" has 



nf 



en 



Fig. 10. Squalus ac, 2G somites, frontal section, inf., the so-called infun- 

 (libulum. 



pressed down to meet the ectoderm. In Fig. 8 the origin of this 

 from the neuropore is strongly suggested, especially as the spot 

 marked neurop. is at the upper border of the lamina terminalis. 

 some distance dorsal to the extreme point to which the preoral ento- 

 derm or premandibular mesoderm ever reaches. In Dr. Neal's 

 preparations which I have studied the entoderm has a different 

 tone from the other tissues and there is a decided difference in the 

 form of the cells and in the size of the nuclei between the preman- 

 dibular mesoderm and the terminal neural crest mesectoderm. 



In embryos of 29 and 30 somites (Figs. 11 and 12) premandibular 



