365 
In the divided encephalon (Fig. 16) it will be seen that the dividing 
lines between segments are no longer present as grooves on the internal 
surface but as ridges. These ridges are absent in the Trout. Sections 
however show that a groove is present along the crests of many of 
these ridges and the plausible explanation is that an intrasegmental 
neural expansion has produced intersegmental ridges and thus elevated 
the primary internal segmental grooves. 
In embryos with 14 somites, 33 hours old (Figs. 17 and 18), the 
segments of the fore- and mid-brains have disappeared. Segment 6 
Fig. 17. Fig. 18. 
Fig, 17. Encephalon of a Chick embryo with 14 somites, 33 hours old. Dorsal 
view. au. vs. auditory vesicles. Letters and figures the same as in Fig. 14. X 50 
diameters. 
Fig. 18, Right surface view of the same encephalon as represented in Fig. 17, 
r dorsal constriction that divides the fore-brain into prosencephalon and thalamencephalon; 
6—11 segments of the hind-brain, Segment 6 is the cerebellum and 7—11 the medulla, 
represents the cerebellum and 7—11 the medulla. Segment 9 is 
wedged-shaped and can be used as a land mark in all subsequent 
stages by means of which its identity in older stages can easily be 
established. 
