THB DEVELOPMENT OJ!" THE MOLE. 115 



the thickening of the vertebral portions of the mesoblast 

 causes the hypoblast cells underlying those structures to be 

 stretched out as it were and flattened (No. 8, fig. 45). 



In Stages e and r this condition may still be seen where the 

 groove is deepest in front of the protovertebrse (fig. 13) ; 

 anteriorly the groove becomes shallower and the hypoblast 

 cells more rounded in consequence (fig. 14), while posteriorly 

 the formation of protovertebrse forces the lateral hypoblast 

 downwards, and the axial hypoblast cells are again thickened 

 (figs. 15 and 17). 



In the region of the sinus rhomboidalis the medullary groove 

 again projects considerably below the level of the peripheral 

 body wall, and, forcing the hypoblast cells downwards also, 

 flattens them. 



This condition in the anterior region and posteriorly below 

 the sinus rhomboidalis is, however, soon modified ; the thick- 

 ening of the peripheral mesoblast and the gradual depression 

 of the body wall brings the lateral portions of the hypoblast 

 on a level with the axial portion throughout the length of the 

 embryo, and at the close of Stage J the cells of the whole 

 layer, wherever it is not converted into the alimentary canal, 

 become rounded. 



The Alimentary Canal. — The first trace of the alimentary 

 canal appears during Stage d {vide No. 8, fig. 46) at the 

 anterior end of the embryo as a short tubular diverticulum. 

 In the paper referred to I described this tube as the noto- 

 chord, an error which I have corrected here and in more detail 

 on p. 118 of the present paper in the section devoted to that 

 organ. 



This structure is indicated in figs. 11 and 13, Stage b. The 

 diverticulum has but a small lumen, and is situated close 

 against the cephalic plate; the cells of which it is formed are 

 columnar. 



Stages G and h witness further changes ; the fore-gut is now 

 considerably longer (fig. 34). It is rounded anteriorly (fig. 

 32), but farther backwards is widened out laterally (fig. 19) 

 and becomes flattened and crescent shaped, the lateral horns 



