54 WALTER HEAPE. 



Stage D. 

 The Medullary Groove, Notochord, and Neurenteric 



Canal. 



The main differences in the superficial appearance in an 

 embryo of this stage of growth are : 



1. The disappearance of the bind knob of the primitive 

 streak and the widening out of that portion of the area. 



2. The great enlargement of the area in front of the primitive 

 streak, and 



3. The appearance in the latter portion of the area of a broad, 

 light-coloured band, the limits of which are at first vague, but 

 which gradually become more emphasised. 



This is the medullary groove which first arises near the 

 anterior end of the primitive streak, and from there extends 

 forwards. 



Fig. 13 represents an embryonic area, in which a shallow 

 medullary groove is formed; the extent of the groove is faintly 

 indicated in front ; at the sides it is more definitely marked 

 off; while behind it abuts upon the anterior end of the primi- 

 tive streak, and terminates abruptly. 



At the junction of the medullary groove and primitive streak 

 a deep pit is visible ; this is the dorsal opening of the neuren- 

 teric passage, which, however, does not appear completely to 

 perforate the blastoderm at this stage. 



The primitive streak extends from the hind end of the me- 

 dullary groove to the edge of the blastoderm, spreading out 

 there into two horns. 



In fig. 14 the medullary groove is more distinctly indicated ; 

 a tongue-shaped band extends anteriorly from the front end of 

 the groove towards the edge of the blastoderm in that direction, 

 and is the anterior end of the thickened axial mass of cells 

 underlying the epiblast. 



The primitive streak meets the opposite end of the medullary 

 groove, and sends a forward prolongation between its divergent 

 walls. 



