472 'Journal of Comparative Neurology and Psychology. 



ectoderm and brain, surrounding the anterior end of the preoral 

 entoderm. Paired sheets or flaps of cells migrating from the ecto- 

 derm at the lips of the neuropore extend in between the ectoderm and 

 brain and surround the anterior part of the preoral entoderm on its 

 under or ectal surface. These sheets of cells resemble in every way 

 the neural crest in other parts of the embi*yo and should be con- 

 sidered as the terminal part of the neural crests. 



In embryos of 24 somites the premandibular cavities are some- 

 what larger and especially longer. The bodies of these somites are 



.en 



k lltm-ec. 



- m 



Fig. S. Squalus ac, 22 somites, frontal section througli the terminal neural 

 crest. X 150. 



in contact with the mandibular somite, are still continuous with the 

 median mass and converge forward and downward like the arms of 

 a letter V to fuse completely with the median mass. In the lateral 

 part of the median mass directly forward from the premandibular 

 somites and behind the "infundibulum" are found the first indica- 

 tions of the anterior head cavities. Many mitoses appear here, 

 especially in the front wall of this cavity. The preoral entoderm 

 continues forward beneath the "infundibulum." As compared with 

 earlier stages, before the formation of the anterior cavities the pre- 

 oral entoderm is very slender and at one point beneath the "infun- 



