92 



EDWARD MCCRADY, JR. 



The cervical flexure. The flexures which play such a con- 

 spicuous part in changing the external as well as the internal 

 appearance of the embryo between stages 26 and 35 seem to 

 be primarily associated with local spurts of growth in the 

 central nervous system. The first flexure is the cephalic, 

 which was described in connection with stage 26. This seems 

 to be due to a lengthening of the mesencephalic plate which 

 causes it to buckle and lift dorsally at the level at which the 



Fig. 29 Photographs and drawing of stage 27. A, ventral view of 16148 after 

 removal of non-vascular chorion showing posterior intestinal portal, and open 

 anterior neuropore. B, drawing of specimen 17141, showing caudal amniotic fold 

 and open otocyst. C, photograph of caudo-ventral view of 17185 showing open 

 anterior neuropore and anterior intestinal portal. 



nuclei of the third and fourth cranial nerves and the nucleus 

 ruber and the substantia nigra will later appear. 



During stage 27 two new growth centers appear in, 1) the 

 alar plates of the mesencephalon, and 2) the cervical cord. 

 The growth of the alar plates (from which the corpora quadri- 

 gemina develop) causes the mesencephalic walls to rise dor- 

 sally and close into a tube before either the prosencephalon 

 or the metencephalon have closed over. The growth of the 

 cervical cord causes the central nervous system to bend and 



