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.) 
x 
SS 
N=) sompi 
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. 
Fic. 54 A.— Transverse section through the head of a 10s embryo. The 
region of the section is near the center of the hind brain. 
Ao., Aorta. End’ec., Endoecardium. End’e. S., Endocardial septum. 
H. B., Hind brain. My’e., Myocardium. p.C., Parietal cavity. Ph., pharynx. 
So’pl., Somatopleure. Spl’pl., Splanchnopleure. v. M., Ventral mesentery. 
The Neuropore. [rom the place where the neural folds first 
meet, the elevation and fusion proceed both forwards and back- 
wards in a continuous fashion (ef. Figs. 59, 61, 65, ete.). 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 12s 
