HEAD-FOLD TO TWELVE SOMITES 



101 



the ectoderm are destined to form the neural crest, a structure 

 of great significance, inasmuch as the series of cranial and spinal 

 ganglia is derived from it. (See following chapter.) 



J'Som'f/. 



] -3 pi' pi. 



«ri>-*c*?*^*c^ 



Fig. 54. — Transverse section through the same embryo a short distance 

 in front of the anterior intestinal portal. For explanation of letters see 

 preceding figure; in addition: Ph., Pharynx. Som'pl., Somatopleure. 

 Spl'pl., Splanchnopleure. v. M., Ventral Mesentery. 



Fig. 54 a. — Transverse section through the head of a 10 s embryo. The 



region of the section is near the center of the hind brain. 



Ao., Aorta. End'c, Endocardium. End'c. S., Endocardial septum. 

 H. B., Hind brain. My'c, Myocardium, p. C, Parietal cavity. Ph., pharynx. 

 So'pl., Somatopleure. Spl'pl., Splanchnopleure. v. M., Ventral mesentery. 



The Neuropore. From the place where the neural folds first 

 meet, the elevation and fusion proceed both forwards and back- 

 wards in a continuous fashion (cf. Figs. 59, 61, 65, etc.). Although 

 the open anterior stretch of the neural tube is very short in com- 

 parison to the posterior open part, it is not until about the 12 s 



