No. 2.] THE MEDULLARY GROOVE. t^^^ 



faint circular depressions that are concave upwards, show even 

 at as early a stage as is represented in Figure 2. [It should be 

 stated for clearness that Figures 2, 3 and 4 represent stages 

 that occur between those designated C and D by Ziegler.'^ I 

 have met with no figures of corresponding stages in any author- 

 ities I^ have been able to consult.] The circular depressions 

 meet in the median line and encroach upon the tongue-like 

 process and it becomes separated from the extreme anterior 

 tip of the head. The infolding started in this form continues, 

 but it does not take place so rapidly in the median line as on 

 each side of it, and it thus happens that the anterior tip of the 

 head and the front end of the tongue-like process are connected 

 by a narrow isthmus, which becomes gradually reduced, and 

 just before its disappearance it forms a very thin connecting 

 plate between these two points (Fig. 3). 



Fig. 2 shows the central tongue-like process continuous with 

 the anterior tip of the head and Figs. 3 and 4 show it (T') 

 after its separation from the latter. 



The optic vesicles are formed in the lateral parts of the 

 depressed region; they are circular in outline and concave 

 from within while externally they form corresponding eleva- 

 tions. In Fig. 3 the head-folds are so much expanded laterally 

 beyond the optic vesicles that these structures seem, at first 

 sight, very much out of place ; but the broad folds, which in 

 the figure are at the side of the vesicles, grow upwards and 

 unite in the median plane, and by this process the head 

 becomes narrow so that the eye vesicles are then laterally 

 situated with reference to the rest of the head. It thus 

 appears that the optic vesicles are formed very early and 

 almost at the extreme anterior tip of the head. They do not 

 begin to move backwards until the anlage of the nose is 

 formed. In Figs. 3 and 4 that part of the cephalic plate 

 which is behind the optic vesicle is all on a higher plane 

 than the anterior part. In Fig. 4 this elevated part of the 

 cephalic plate is divided into concentric ridges (i, 2, 3) and 

 furrows. I shall reserve a more detailed description of the 

 morphology of these structures for my forthcoming pubb'ca- 

 tion. 



