HEAD-FOLD TO TWELVE SOMITES 103 
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Fic. 57. — Later stage of the neural folds. Section through the head of an 
embryo of 2-3; corresponding to about the future mid-brain region. 
Coel., Coelome. g. C., Germinal cells. med. pl., Medullary plate. Mes., 
Mesoblast. n. F., Neural fold. n. Cr., Neural crest. N’ch., Notochord. som. 
Mes., Somatic layer of mesoblast. spl. Mes., Splanchnie 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 hes at 
the stage of 10 somites a short 
distance in front of the ante- Fic. 58. — Ventral view of the head 
rior end of the oral plate in region of an embryo of 5 somites, 
the region of the future re- re in eas with transmitted 
: a8 ie ’ ight. x 30. 
cessus opticus (Fig. 62). (Go- a. @. v., Amnio-cardiae vesicle. 
ronowitsch calls the anterior a. i. p., Anterior intestinal portal. 
F.G., Fore-gut. My’e., Myocardium. 
es N’ch., Notochord. n.I*., Neural fold. 
rior; His divided it into twos 2,s 4, Second and fourth somites. 
fissure, sutura cerebralis  ante- 
