407 



border, we shall see that the elevations and constrictions are sub- 

 stantially as they have been in all the previous stages, but, those 

 along the upper margin, no longer correspond with them. In all the 

 preceding figs., the boundaries of the segments correspond on both 

 upper and lower margins. In fig. 10, however, the upper margin 

 shows elevations just above the constrictions on the lower margin. 

 These newformed elevations become very quickly prominent, while 

 the segments along the lower margin lose their individuality, and, 

 the segmented area, becomes more and more an irregular sinuous 

 band with crests upon its upper margin. The entire line of segments 

 become indistinguishable, but, if they be studied in stages immediately 

 following that represented in fig. 10, it will be the crests along the 

 upper margin that first catch the eye. If the observations are made 

 from above, these crests are seen to be transverse folds on each side 

 of the medulla, and when counted, will, of course, be one less than 

 the original segments. It is only by viewing them from the side, 

 and comparing them with earlier stages, that we shall be able to 

 identify the boundaries of the original segments. 



Just what is taking place during the appearance of the crests is 

 not now clear to me. I have, heretofore, assumed that it signified a 

 union of the original segments, the anterior half of one, with the 

 posterior half of the segment lying just in front of it, but, at present, 

 I am inclined to question that interpretation. A further study of 

 sections may throw light on the question. 



The crests on the upper margin are between two "neuromeres" 

 as designated by Ore, and they correspond in position to his inner 

 "ridge". 



It is during the appearance of the crests on the upper margin, 

 that I have first noted, from surface views, the nerves. The fibres 

 of the fifth nerve (fig. 9 n. v) arise, as Miss Platt has described 

 them in the chicken: — from the concavity (on the lower margin) 

 between two neuromeres. The scope of the present paper will not 

 carry me further into discussion of the relations of the nerves. 



It will be interesting to note, incidentally, in this fig. the very 

 large development of the mid-brain over that in fig. 8, and the con- 

 sequent crowding forwards of the first accessory optic vesicle. The 

 latter structure is also much reduced in size and with its fellow is 

 in the region of the thalamencephalon. 



