HEAD-FOLD TO TWELVE SOMITES 



103 



Coef 



somJfes. 





'Coel. 



FIG. 57. Later stage of the neural folds. Section through the head of an 

 embryo of 2-3 s; corresponding to about the future mid-brain region. 



Coel., Coelome. g. C., Germinal cells, med. pi., Medullary plate. Mes., 

 Mesoblast. n. F., Neural fold. n. Cr., Neural crest. N'ch., Notochord. som. 

 Mes., Somatic layer of mesoblast. spl. Mes., Splanchnic layer of mesoblast. 



anterior end rapidly grows forward beyond the anterior end of 

 the fore-gut. The floor of the neural tube does not, however, 

 take part in this extension, the consequence being that the sum- 

 mits of the neural folds form 

 arching knees extending in front 

 of the original anterior end of 

 the medullary plate (Figs. 51 

 and 52). The extreme anterior 

 end of the neural tube formed 

 in this way has a ventral as well 

 as a dorsal defect, and when it 

 closes there is a ventral as well 

 as a dorsal suture. The ventral 

 end of this suture marks the 

 original anterior end of the me- 

 dullary plate, and this lies at 



F.ff. 



*././>. 



the stage of 10 somites a short 



distance in front of the ante- FIG. 58. Ventral view of the head 



rior end of the oral plate in re S ion of an embryo of 5 somites, 



the region of the future re- 



cessus opticus (Fig. 62). (Go- 



drawn in balsam with transmitted 

 light, x 30. 

 a. c. v., Amnio-cardiac vesicle. 



ronowitsch calls the anterior a. i. p., Anterior intestinal portal, 

 fissure, sutura cerebraKs ante- %%$53S^*SS^ 

 rior; His divided it into two s 2, s 4, Second and fourth somites. 



