128 WALTEE HE APE. 



and forms the medullary plate (fig. 15) while the lateral por- 

 tions become gradually still more reduced in thickness, until in 

 those portions of the embryo where the medullary groove has 

 attained its greatest depth prior to its conversion into a canal 

 the lateral epiblast is formed for the most part of a single row 

 of somewhat cubical cells, continuous, without modification, 

 either over the vesicle or across the embryo as the inner fold 

 of the amnion (fig. 17). 



Where the lateral epiblast joins the wall of the medullary 

 groove there is now an abrupt transition from the columnar 

 cells lining the latter to the cubical cells of the former. 



Subsequently, Stages h. j., in that portion of the embryo 

 where the neural canal is formed, the closure of the medullary 

 groove causes the approximation of the lateral portions of the 

 epiblast, which fuse, and thus form a continuous layer across 

 the embryo. The cubical epiblast cells at the same time become 

 much flattened on the dorsal surface of the embryo (figs. 26, 29, 

 and 47), while (1) in the trunk, the cells of that portion of epi- 

 blast which overlies the somatic mesoblast remain cubical 

 (figs. 26, 27, 29, and 47) ; and (2) in the anterior region, the 

 cells of certain portions which either give rise to sensory 

 structures (figs. 25 and 46), or which surround externally the 

 visceral arches (figs. 23 and 46) assume again a columnar 

 form. 



In that region of the trunk where the medullary canal is 

 still open the lateral epiblast cells remain as before, cubical. 



The Medullary Groove. — At the commencement of Stage e 

 a deep medullary groove exists about the middle of the embryo ; 

 anteriorly and posteriorly it is shallower however, finally ter- 

 minating in the latter direction upon reaching the anterior end 

 of the primitive streak, while in the former direction all trace 

 of the groove is lost some considerable distance behind the front 

 end of the embryo. 



Beyond the anterior end of the medullary groove the epiblast 

 is thickened to form the " cephalic plate." 



Fig. 1 is a transparent view of an embryo with three proto- 

 vertebrse, and shows the relations above mentioned ; 1 have 



