420 CHARLES HILL, 



cephalon of an embryo with IG somites 33 hours old. The transverse 

 groove that separates segment 6 from segment 7 has become more 

 distinct, otherwise the segmental characteristics are the same as those 

 described in Figs. 32 and 33. 



In embryos with 20 somites 3 hours older a very marked 

 change has occurred in the region of the hind-brain. In the dorsal 

 view (Fig. 36) the segments of the medulla have expanded laterally, 

 especially segment 8 which now represents the widest part of the 

 hind-brain. The dorsal surface of this region is smooth and unseg- 

 raented. Of the 7 transverse segmental grooves that are now 

 present, only two (c and e) completely encircle the brain. These, as 

 observed before, mark the anterior and the posterior limits of the 

 mid-brain. 



d) Embryos 40 to 100 hours old. 



Figs. 37 and 38, Plate 3, represent two views of the encephalon 

 of an embryo 43 hours old. The encephalon has partly turned upon 

 its left side. The expanded condition of the medulla reveals the 

 presence of a broad cavity, the 4th ventricle, lying beneath a thin 

 unsegmented roof. The 6 segments of the medulla present the same 

 characteristics as previously described in Fig. 36. When the embryo 

 has turned completely upon its left side (Fig. 39), the position of the 

 4th ventricle with its thin unsegmented roof is more clearly defined 

 Segment 6, the cerebellum , has broadened and from this time on 

 continues to differentiate from the other 5 segments of the hind- 

 brain. 



In embryos 50 hours old, the segmentation of the medulla is 

 practically unchanged (Fig. 40). The cavity of the 4th ventricle has 

 expanded laterally and its thin unsegmented roof has increased in 

 extent. A transverse dorsal groove (s) has appeared dividing the 

 thalamencephalon into two nearly equal divisions, and at this age 

 therefore three distinct divisions can be recognized in the primary 

 fore-brain, viz., a distal portion, the cerebrum and two proximal 

 portions that represent the two divisions of the 'tween brain. 



The segmental condition just described remains practically un- 

 changed in all essential features during the entire third day of in- 

 cubation. Fig. 40 may therefore very well represent this period. 



During the last half of the fourth day of incubation the seg- 

 mentation of the medulla very rapidly grows indistinct and ultimately 

 disappears. Fig. 41 represents the encephalon of an embryo 80 hours 



