Johnston, Forcbrain Vesicle in Vertebrates. 477 



this stage onward Neal's figures 12, 13, etc., show the distribution 

 of neural crest mesectoderm ahnost correctly. The mesectoderm 

 lying in the region between the optic vesicle and somite 1 should 

 be continued as a thin sheet between the brain and ectoderai beneath 

 the neuropore and the origin of all this part from the terminal part 

 of the neural crest should be taken into account. 



tm-ei 



Fig. 13. Squalus ac, 39 somites, froutal section through the anterior head 

 cavities, showing their relation to the premaudibular somite (left side) and 

 the mesectoderm derived from the terminal neural crest. The connection 

 between n. thalamicus and terminal neural crest shows on the right side. 

 X 150. 



The relations of the anterior head cavities to somite 1 and to the 

 mesectoderm remain essentially unchanged up to embryos of 50 

 somites. Figs. 12, 11, 13, 14 show these relations in frontal sec- 

 tions of embryos with 29, 30 and 39 somites. The brain, mesoderm 

 and mesectoderm with the cranial ganglia have been modelled in an 

 embryo with 42 somites and Fig. 15 shows these relations as seen in 

 the model. 



