No. 2.] THE MEDULLARY GROOVE. 369 



as lateral expansions, and the newly formed groove (Fig. E', in.g) 

 is due to the upgrowth and approximation of the medullary folds. 

 The Stage usually characterized as marked by the " first 

 appearance of the medullary groove " should be placed con- 

 siderably later. As we shall presently see the medullary folds 

 are not yet formed at Ziegler's Stage C. 



The surface changes that accompany those shown by the 

 sections are partly represented in Figs, i, 2, 3, 4, 5 and 6. 

 These figures are to be used later for another purpose. They 

 are not chosen to illustrate especially the different phases of 

 medullary groove formation, and there is, therefore, no precise 

 correspondence between them and the sections. The appearance 

 of the embryo from above during the first stage of the median 

 furrow is well known — it is not shown in my figures. It is, 

 however, that stage designated "B" by Balfour, and is just 

 after the body of the embryo begins to appear by the con- 

 crescence of the embryonic rim. Fig. i represents approximately 

 the stage from which the sections B and B' are taken. The 

 median furrow in this period of growth is rather deep near the 

 middle of the embryo, but is shallow in front and behind. 

 Fig. 2 shows a slightly younger stage than the embryo from 

 which the sections C, C and C" were taken. The median 

 furrow in this period of growth becomes obliterated with the 

 exception of a very abreviatcd portion of it located in the mid- 

 dorsal region. My material does not enable me to say at 

 present whether this remnant of the median furrow entirely 

 disappears or not. I have found the surface study of embryos 

 in this stage very deceiving as to the depth, or even the presence 

 of the median furrow. The central part of the embryo is 

 occupied by the gastrular cavity, and its anterior extension, the 

 archenteron, and this extensive cavity beneath the surface 

 lights up in such a way as to create the impression of a well 

 marked median furrow when there is none ; but, of course, the 

 sections are conclusive on this point. 



The manner of formation of the medullary folds is very 

 unusual — so far as I know it is unlike that of any other 

 animals whose embryology has been recorded. Glancing again 

 over the figures indicated by the letters B', C, C" and E' we shall 



