THE DEVELOPMENT OF THE MOLE. Ill 



also figured three transverse sections, which indicate the struc- 

 ture and form of (1) the cephalic plate (fig. 13) ; (3) the groove 

 in its anterior portion (fig. 14) ; and (3) the groove in its 

 posterior portion (fig. 15). 



In fig. 13 the thick cephalic plate is shown, becoming folded 

 ofi^ from the yolk-sac; fig. 14 is taken from the region in front 

 of the protovertebrsBj and depicts the wide and shallow groove, 

 the wall at the bottom of which is considerably thinner than 

 at the edge of the groove ; and in fig. 15, taken from the region 

 of the second protovertebra the medullary groove is V-shaped, 

 and the columnar cells of which it is formed pass abruptly into 

 the lateral epiblast cells, thus indicating the extent of the 

 " medullary plate." 



At the hind end the wide and shallow medullary groove 

 forms the so-called " sinus rhomboidalis." A section through 

 this region of an embryo during Stage r is shown in fig. 18. 



At the close of Stage e the groove has considerably increased 

 in length, and during Stage f it reaches to the anterior end 

 of the embryo (figs. 3 and 16). The latter figure is a trans- 

 verse section through the anterior end, and shows — 



(]) The median medullary groove. 



(3) The commencement of the curvature upwards of the 

 lateral portions of the cephalic plate and the formation of the 

 two "optic grooves" {op. gr.), seen in surface view in fig. 4, 

 which give rise when the neural canal is closed, to the optic 

 vesicles. 



The Medullary Canal. — The medullary plate is now sharply 

 marked off from the lateral epiblast from a considerable dis- 

 tance in front of the first protovertebra backwards to the pos- 

 terior end of the embryo, and the groove itself commences to 

 close in the region of the protovertebrse. 



The closure is efi'ected by the approximation of the peri- 

 pheral edges of the medullary plate, a sharp angle being thus 

 formed at the junction of the lateral epiblast with the edge of 

 the plate (fig. 17). 



The closure commences at a late period of Stage g in the 

 region of the first provertebra, extending thence forwards and 



