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. 
L£nt. & 
Fic. 52 A. — Median longitudinal section of the head, stage of 4s. The sec- 
tion passes through the length of one of the neural folds just behind the 
anterior end. (Cf. Fig. 51.) 
a. i. p., Anterior intestinal portal. Ect., Eetoderm. Ent., Entoderm. 
F. G., Fore-gut. H. F., Head-fold. Mes., Mesoderm. Mes. H. C., Meso- 
blastic head cavity. n. F., Neural fold. or. pl., 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 
