HEAD-FOLD TO TWELVE SOMITES 



107 





ceph. Mes 



£nd'c.5. 



V.o.m . 





Kifi. 





d.i.p. ■ 





B 



-S.B. 



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 notbchord, and at the lowest level, 



the neural tube. 



a. c. s., Anterior cerebral suture. Inf., Infundibulum. 

 M. A., Mandibular arch. p. C, represents the anterior 

 boundary of the parietal cavity, or. pi., Oral plate. 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 



