444 CHARLES HILL, 



(c', e') have appeared on this surface. They mark the posterior limits, 

 respectively of fore- and mid-brains. The dorsal margins give evidence 

 of 3 segments in the former (2, 2, 3) and 2 segments in the latter 

 (4, 5). 



Pig. 25. Encephalon of an embryo with 6 somites, 20^/3 hours 

 old. Dorsal view. The neural groove is closing, c and e mark the 

 posterior limit of fore- and mid-brains. As in Fig. 24, 3 segments can 

 be counted in the former (i, 5, 3) and 2 in the latter (4, 5). 



Fig. 26. Right surface view of the same encephalon as represented 

 by Fig. 25. The second encephalic groove affects the base of the optic 

 evagination. The latter appears divided. 



Fig. 27. Left half of the same encephalon represented by Figs. 25 

 and 26. Internal surface exposed to view. Internal segmental lines 

 appear as transverse ridges (c', e', g' etc.). 11 encephalic segments are 

 distinctly present along the thick dorsal margin (1 — 11). 



Fig. 28. Encephalon of an embryo with 7 somites, 26 hours old. 

 Dorsal view. The 8 anterior segments can be detected along the 

 dorsal margin. Laterally the optic expansions have obliterated these 

 segments. 



Fig. 29. Right surface view of the same encephalon as repre- 

 sented by Fig. 28. In this position all of the neural segments can be 

 seen (i — IT). The component segments of the fore-brain (2, 2, 5) and 

 of the mid-brain (4, 5) are well defined. 



Fig. 30. Encephalon of an embryo with 11 somites, 29 hours old. 

 Dorsal view. The 3 anterior segments have disappeared. c re- 

 presents the posterior limit of the fore- brain. It will be observed that 

 the optic expansion is now confined to the distal portion of this brain. 



Fig. 31. Right surface view of the same encephalon as represented 

 in Fig. 30. The dividing groove between the 2 segments of the mid- 

 brain (4, 5) is about to disappear. 



Fig. 32. Encephalon of an embryo with 14 somites, 33 hours old. 

 Dorsal view. The mid-brain now appears unsegmented. Optic evagin- 

 ations are now connected with the anterior portion of the fore-brain. 

 The dividing line between segments 6 and 7 cannot be seen in this 

 view. 



Fig. 33. Right surface view of the same encephalon as repre- 

 sented in Fig. 32. The dividing line between segments 6 and 7, which 

 does not appear in the dorsal view, can be seen in this position. Seg- 

 ment 6 represents the cerebellum. Segment 9 has become wedge- 

 shaped, and in older stages serves as an anatomical landmark. 



Fig. 34, Encephalon of an embryo with IG somites, 36 hours 

 old. Ventral view. Dividing line between segments 6 and 7 is very 

 faint. 



Fig. 35. Right surface view of the same encephalon as repre- 

 sented in Fig. 34. Dividing line between segments 6 and 7 can be 

 better seen in this position than in the ventral view (Fig. 84) 



