53 



ween the stages represented in figs. 1 and 2 respectively the cells of the 

 neural crest are increased in number by division, and the mesecto- 

 derm of which they form part has also been increased by the addition 

 of cells further proliferated from the ectoderm, not promiscuously, as 

 seems to be the case in the bird, but in specialized regions and de- 

 finite order of succession. These united cells continue their downward 

 migration until, at the time when four pockets from the alimentary 

 canal have fused with the ectoderm, prior to the formation of the 

 gill clefts, the mesectoderm occupies the position indicated in fig. 2. 

 I have followed the increase and migration of these cells between 

 the stages given in figs. 1 and 2, through a series of twenty recon- 

 structions made with 

 the camera from sec- 

 tions 4 / 300 mm i Q 

 thickness. Although 

 these sections are not 

 thin, their thickness 

 is approximately the 

 diameter of a nucleus, 

 and three such sec- 

 tions may be required 



Fig. 1. aud auditory epithelium, br floor of brain, dl — d2 ant and post, divisions 

 of neural crest, ect ectoderm as it passes over the yolk , opt optic vesicle still widely 

 open to the brain. (Only one layer of cells is represented in the diagrams, where two, 

 three, or even four exists.) 



Fig. 2. F attachment 

 of Gasserian ganglion, 

 VII attachment of fa- 

 cial ganglion, IX glos- 

 sopharyngeal ganglion 

 forming, Jsome of the 

 vagus cells, aud audi- 

 tory vesicle, br floor 

 of brain, 1 Jj — 2 br 

 first and second bran- 

 chial clefts, hy-br hyo- 

 branchial cleft, hy-m 

 hyo-mandibular cleft, 

 I lens , 61 olfactory 

 epithelium , x point 

 where cells migrating 

 from the anterior di- 

 vision of the neural 

 crest meet those migra- 

 ting from the posterior 

 division. 



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