47^ "Journal of Comparative Neurology and Psychology. 



cavities extend forward at the sides of the "infundibular" region in 

 the position in which they were first described by Miss Piatt. Ex- 

 cept where they are in contact with the premandibular somite the 



Fig. 12. Squalus ac, 29 somites, froutal sectiou through the premaudibiihir 

 and anterior head cavities. These cavities are slightly more advanced in 

 development than in the 30 somite embryo shown in Fig. 11. x 150. 



anterior somites are almost completely ensheathed by the mesec- 

 toderm derived from the region of the neuropore. The mesectoderm 

 sheet derived from the thalamic nerve rudiment has now come down 

 behind the eye and met with this which lies beneath the eye. From 



