372 LOCY. [Vol. VIII. 



region. As before indicated, he consistently refers to the 

 median furrow as the medullary groove. 



Figs. A, B, C, D, E, F and D" show certain characteristic 

 stages in the formation of the medullary folds in the region 

 of the cephalic plate, and, also, certain steps in their upward 

 growth to form the medullary groove. The first five of these 

 sections are taken from the same embryos that afforded the 

 sections already described. The same letters indicate in every 

 case the same embryos, and the marks (' and ") attached to 

 the letters serve to indicate the different regions of the body 

 from which the sections are taken. 



Fig. A shows a section through the head region before any 

 changes have occurred looking towards the formation of the 

 medullary folds. It will be remembered that further back 

 in this same embryo (Fig. A', (/".), the median furrow is well 

 marked. 



In Fig. B we have a faint indication of the median furrow ; 

 on the sides are seen lateral thickenings of the medullary plate 

 (;;{/".) which become transformed into the medullary folds. 



Fig. C is an interesting one; it shows the position of the future 

 groove occupied by an eminence {e^. The medullary folds are 

 far removed from the median plane ; they are unusually wide 

 and have the characteristic (at this period) ventral curvature. 

 A pencil point in following the outline of this section from the 

 outer margin of the medullary folds, will mount in passing 

 towards the median line, and will reach a position on the 

 summit of the central eminence, that is more than one-half the 

 full vertical axis of the embryo above the medullary folds. Yet 

 in subsequent growth changes of such a nature take place, that 

 the medullary folds become elevated, and, arching above the 

 median part they join some distance above it. 



Fig. D affords a contrast with Fig. D", — which is taken 

 from the same embryo near its anterior tip — showing the con- 

 dition of the medullary folds, at this stage, in the anterior (D") 

 and the posterior (D') parts of the cephalic plate. 



Fig. F. shows a stage in which the medullary folds are grow- 

 ing upwards, and have risen a little above the horizontal 

 plane. 



