Johnston, Forebrain Vesicle in Vertebrates. 471 



a conspicuous folding upward of the neural plate in fhe middle line 

 so that a prominent keel appears in the floor of the neural tube 



In embryos of IT somites the preoral entodenn comes into slight 

 but actual contact with the bridge between ectoderm and brain at the 

 lower border of the neuropore. This is indeed the condition which 

 exists from the time the head fold begins to form (Fig. 6). 



r.n. 



:\ 



po 





en 



f 

 '( 



\ 



Fig. 7. Sqiialus ac, IS somites, frontal section tlirougb tlie premandibular 

 somite and median mass. The right side of the section is a little further 

 dorsal than the left and the cavity of the somite shows only on the right. 

 X 150. 



In embryos of 18 somites the cavity of the premandibular somite 

 (som 1, V. Wijhe) is first seen in the lateral part of the undifferen- 

 tiated mass above described (Fig. 1). The median part of this mass 

 maintains the relations just described. 



In embryo of 19 somites the mesoderm and preoral entoderm show 

 no change but a very noteworthy structure has appeared between 



