o/' 



LOCV. [\'0L. VIII. 



see how the medullary folds are formed in the cervical region of 

 the embryo. Fig. B' shows that the cells of the medullary 

 plate are much thickened laterally, and, since the base of the 

 body has become more narrow than the top, the thickened 

 edges project beyond it (Fig. B', w/.). This is quite different 

 from the preceding figure (A') in which the base is the broadest 

 part of the embryo. In Fig. C we see that the thickened 

 edges of the medullary plate have been transformed into the 

 medullary folds {inf.) which are expanded laterally beyond the 

 body, and are also ventrally curved. When completely formed 

 the medullary folds constitute wing-like expansions running 

 along each side of the body of the embryo and. gradually 

 fading out behind. 



In the anterior head region the lateral expansion of the folds 

 is greatest (Figs. C and D", inf.), and behind the cephalic plate 

 they become narrower as we follow them backwards. This 

 decrease in width of the folds is well shown by comparing 

 Figs. C and C" which are sections of the same embryo drawn 

 to the same scale; but the section C" is considerably behind the 

 section C. Behind the section C" the folds show relatively 

 the same amount of lateral expansion, to a point about seven 

 eighths the length of the whole embryo. Back of that point, 

 the concrescence of the embryonic rim is progressing, and the 

 condition of the medullary folds in that part of the body 

 requires a further account illustrated by sections, which I shall 

 give at a future time. 



There is one region, immediately behind the much ex- 

 panded cephalic plate that is deserving of especial mention. 

 Here, the medullary folds are abruptly folded downwards for a 

 considerable space and come almost into contact with the sides 

 of the body. Fig. G represents a section through this particu- 

 lar region, and the position of the bend in the medullary folds 

 is indicated by the shading in Figs. 2, 3, 4 and 5 {bd.). This 

 abrupt bending of the medullary folds makes its appearance as 

 soon as the cephalic plate is formed and it persists through 

 several stages of growth. Just in front of the bend on each 

 side the cephalic plate becomes raised into eminences (Fig. 

 3,x; Fig. 4,3). 



