40 J. GRAHAM KKRE. 



Medullary j,'roove. m.k. ^Jedullary keel. m.s. Spinal cord. myoc. 

 Myoccele. n. Notochord. p.n. Pronephros. 

 Fig. 19. — Transverse section tiiroiigli neural rudiment. Stage 18. 



n.c. Involution of outer surface of ectoderm to form a vestigial neural 

 canal. 



