HEAD-FOLD TO T\VEL^'E SOMITES 



107 



a.cs. 



ceph. Mes 



End'c.5. 





d.l.p. 





s.z. 



Pig 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. c. s., Anterior cerebral suture. Inf., Infundibulum. p. C, 



represents the anterior boundary of the parietal cavity, or. pi., 



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 



