HEAD-FOLD TO TWELVE SOMITES 107 
Fig. 62.— The head of the same embryo from beneath more 
highly magnified. In this drawing an attempt is made to 
show different levels of the embryo superposed: thus the 
heart is uppermost in the figure, beneath this the fore-gut 
(F.G.), beneath this the notochord, and at the lowest level, 
the neural tube. 
a.¢.s., Anterior cerebral suture. Inf., Infundibulum. p.C., 
represents the anterior boundary of the parietal cavity. or. pl., 
Oral plate. v. Ao., Ventral aorta. Other abbreviations as 
before. 
is no clear distinction between brain and cord at. first, the one 
passing without any anatomical landmark into the other. Now 
the brain is the central nervous system of the head, so it is not 
until one can determine the posterior boundary of the embryonic 
head that it becomes possible to determine the hind end of the 
