No. 2.] THE MEDULLARY GROOVE. 



373 



In Fig. E, the medullary folds are elevated into nearly a per- 

 pendicular position, their upper margins are still slightly rolled 

 ventrally. 



The medullary canal is produced from the medullary groove 

 in the usual way, by the coalescence of the folds in the median 

 plane. In the process of closing, the walls of the groove come 

 together first in the extreme anterior tip of the head in front 

 of the eyes and they unite a very little in this region : they are 

 then approximated in the cervical region and the closing of the 

 groove progresses both anteriorly and posteriorly but it does, 

 not close throughout the length of the embryo by a continuous 

 extension of this union. In the posterior dorsal region a new 

 point of contact is made considerably behind the point reached 

 by the backward growth of the first cervical union, and, in this 

 way, an open sinus is formed between the new point of contact 

 and the canal already formed in front of it. This sinus closes 

 gradually but after it is completely closed, the medullary canal 

 is still open behind that region by a continuous posterior slit as. 

 Ziegler has shown in his Fig. 8.^ 



In the meantime the closing of the groove in front progresses^, 

 even more slowly, and, after the posterior sinus is completely 

 closed, that region of the brain lying above the abrupt bend 

 of the medullary folds, already described, is conspicuously open 

 and the margins of the medullary folds are ventrally rolled. 

 In front of this, between the eyes, there is a distinct neuropore 

 that closes last of all. 



The decided tendency, as recorded above, of the rudiments 

 of the dorsal nerve cord to grow ventrally when they are first 

 formed is suggestive, but I shall indulge in no speculations at 

 present as to what may be the theoretical bearing of these 

 facts. 



In addition to the unique features of medullary groove for- 

 mation, there is to be noted in my preparations several inter- 

 estmg points as to the formation of the cephalic plate and its 

 transformation into the head of the embryo. 



In an early stage of growth the anterior tip of the body 

 from which the cephalic plate is to be formed shows na 



1 Lc. p. 8i. 



