30 CORA J. BECKWITH. 



Explanation of Plate I. 



Fig. I. Embryo of Amia in which there is a thickening (t/i.) of the mesectoderm 

 or neural crest on the neural tube just back of the hind brain. X 2 °- 



Fig. 2. Section through thickening along the line a-b, Fig. I showing neural 

 crest and auditory invagination, a. in., auditory invagination ; ec, ectoderm ; en., 

 entoderm ; n.c, neural crest ; n.t., neural tube. X I ^7- 



Fig. 3. Embryo of Amia showing the division of the thickening (t/i.) into two 

 lobes. X 2 °- 



Fig. 4. Section along the line a-b, Fig. 3, showing the gradual merging of the 

 neural crest into the surrounding mesoblast. ec. , ectoderm ; en., entoderm; n.c, 

 neural crest ; n.t., neural tube. X ID 7- 



Fig. 5. Embryo of Amia showing the extension of the neural crest into the 

 spiracular and first branchial arches, ad.org., adhesive organ; a.p., auditory pit ; 

 1st b.a., first branchial arch; h.a., hyoid arch; m.a., mandibular arch. X 2 °- 



FlG. 6. Section through line a-b, Fig. 5, showing the constriction of the auditory 

 invagination from ectoderm. a. in., auditory invagination; ec, ectoderm; en., 

 entoderm ; n.t., neural tube; ntc, notochord. X I ^7- 



