406 CHARLES HILL, 



of the neural axis. The first iudication of a true neural canal has 

 appeared as a fine median cleft dividing the neural axis into lateral 

 halves. Again the dorsal groove (/), that marks the posterior border 

 of the cerebellum, is deeper than any of the other encephalic con- 

 strictions. A lateral neural expansion has taken place in this vicinity, 

 which with the formation of the neural canal, leaves a thin trans- 

 parent roof covering this depression. 



The 6 segments in front of the constriction if) have become 

 more distinct. The dorsal depression (c), which separates the primary 

 fore-brain from the mid-brain, can be identified by its position 

 which, as in sketch A is dorsal to the middle of the lateral eyes, and 

 also by the fact that it is deeper than the other segmental con- 

 strictions of this region. The segments otherwise are both uniform 

 and constant and resemble those of the medulla, except in degree of 

 distinctness. 



Thus far these segmental grooves have been described as though 

 they were confined exclusively to the dorsal region of the encephalon. 

 In living embryos, 16, 17 and 18 days old, I have been able to ob- 

 serve them only in this region. In older embryos, however, both in 

 living and dissected specimens, these grooves can be traced ventrally 

 around the encephalon. The many difficulties involved in dissecting 

 them in the early stages render it possible that in embryos 16 — 18 

 days old they encircle the encephalon just as they do in older forms, 

 but partly on account of their delicacy because the cephalic mesoderm 

 is more dense towards the ventral region of the encephalon, I have 

 only been able to observe them along the dorsal zone. For like 

 reasons I deem it very probable that these segments are present in 

 even younger embryos than 16 days old. 



In hardened embryos, 19 days and older, the brain walls have 

 been laid bare by complete removal of the cephalic mesoderm and 

 the brain thus exposed has been studied by reflected light. Fig. 1, 

 Plate 28, represents the right profile view of the encephalon of an 

 embryo 19 days old, with 29 somites, prepared in this manner. The 

 thin transparent roof of the medulla is removed. Fig. 2 is a dorsal 

 view of the same specimen. In this case it is clear that the trans- 

 verse grooves pass externally down the lateral walls and ventrally 

 around the whole neural axis. 



Having removed the large lateral eyes and the cephalic mesoderm, 

 the five anterior grooves are much better observed than is possible 

 by the study of the living embryo. The auditory vesicle in this 



