HEAD-FOLD TO TWELVE SOMITES 



99 



formed, a short distance back of the anterior end of the medullary 

 plate (Figs. 50 and 51). The posterior ends of the neural folds 

 do not, at this time, reach the region of the first somite. The 

 region where the neural folds first come in contact corresponds 

 approximately with the region of the future mid-brain, or ante- 

 rior part of the hind-brain. 



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Fig. 52 A. — Median longitudinal section of the head, stage of 4 s. The sec- 

 tion passes through the length of one of the neural folds just behind the 

 anterior end. (Cf. Fig. 5L) 

 a. i. p., Anterior intestinal portal. Ect., Ectoderm. Ent., Entoderm. 

 F. G., Fore-gut. H. F., Head-fold. Mes., Mesoderm. Mes. H. C, Meso- 

 blastic head cavity, n. F., Neural fold. or. pi., Oral plate. 



The process of closure itself is essentially the same in all 

 regions of the neural tube. Each neural fold has two limbs: an 

 inner thick limb, belonging to the medullary plate, and an outer, 

 thin limb, continuous with the general ectoderm (cf. Fig. 68 B). 

 When the folds of opposite sides come in contact, the inner limbs 

 of the two sides become continuous with one another, and also 

 the outer limbs, the ectoderm then passing continuously over a 

 closed neural tube. 



Certain cells in the suture and in the walls of the tube next 



