504 HILL. [Vol. V. 



the dorsal surface and somewhat from in front. In the medulla 

 there is represented only the floor, which shows a series of five 

 parallel, transverse grooves, — (7), (8), (9), (10), (11). These 

 grooves are broader and deeper towards the middle line, and 

 become gradually narrower and shallower towards each side. 

 They separate five transverse ridges, that are higher at the 

 middle line and lower or less prominent towards each side. 

 A longitudinal groove, the ventral part of the neural canal, 

 passes along the median line of the floor of the medulla, and 

 separates each transverse groove and ridge into two equal lat- 

 eral parts. The auditory vesicles (A) lie on each side of the 

 medulla, a little in front of the groove marked 10. Each 

 one of these grooves marks the median line of a neuromere, 

 as defined by Orr in Lacertilia, while lines drawn midway 

 between these grooves, and parallel to them, indicate the 

 limits between the individual neuromeres of Orr. Thus each 

 of the ridges here described is equivalent to the posterior 

 half of one of the neuromeres of Orr together with the ante- 

 rior half of the next neuromere posterior to it. I adopt the 

 nomenclature of Orr and describe these neuromeres merely 

 for the purpose of fixing definitely this stage of development, 

 and, for the present, do not wish to be understood as having 

 accepted his interpretation of them. 



The cerebellum (C) extends laterally farther than the medulla, 

 and in the figure, owing to the point of view, appears to be 

 partly covered by that part of the brain anterior to it. In its 

 floor there is but one groove (6), which resembles those of 

 the medulla. Only a part of this groove appears in the figure. 



That part of the brain in front of the cerebellum tapers 

 anteriorly ; so that, in a dorsal view, it has the outline of a 

 pear. The sides of this anterior pear-shaped portion are con- 

 cave ; and in these concavities, close against the brain, are 

 the two optic vesicles. Nothing was seen, at this stage, to 

 justify a division of this part of the brain into primary fore- 

 brain and mid-brain. Four transverse grooves — (1), (2), (4), 

 (5), an anterior pair and a posterior pair — mark its base. The 

 distance between these two pairs is considerable ; and while in 

 this figure no groove is shown in the wall between these two 

 pairs, in other embryos a fifth very small groove was seen in 

 the position indicated by (3). 



