Developmental history of primary segments of the vertebrate head. 419 



portion of the cncephalou presents 3 large divisions or vesicles, which 

 it must be remembered are not simple but compound. 



c) Embryos with 7—20 Somites (Ages 27-40 hours). 



After the complete closure of the neural groove in the cephalic 

 region all evidence of metamerism in the primary fore-brain has dis- 

 appeared, and the distal portion of the neural axis has thus the ap- 

 pearance of a single simple vesicle. The optic evaginations are 

 gradually being confined to the anterior portion of the encephalon. 

 In a dorsal view of an embryo with 11 somites 29 hours old (Fig. 30), 

 the 3 large distal encephalic vesicles appear unsegmented but, in 

 the profile view (Fig. 31) the same encephalon shows, on its external 

 surface, a transverse groove that divides the middle vesicle or mid- 

 brain into 2 segments (segments 4 and 5), and also a second groove 

 that divides the vesicles caudad to this into 2 segments (segments 

 6 and 7). Both of these grooves are restricted to the lateral and 

 ventral portions of the encephalon. As mentioned in the description 

 of Figs. 28 and 29, segment number 6 represents the cerebellum and 

 number 7 is the anterior segment of the medulla. The remaining 

 segments of the medulla {8—11) have become very distinct and their 

 dividing lines pass entirely around the neural axis. 



In embryos with 1 4 somites, 33 hours old (Figs. 32 and 33), the 

 mid-brain is unsegmented. The groove that forms the dividing line 

 between segments 6 and 7 (Fig. 33), has almost disappeared. In later 

 stages, however, this constriction becomes again distinct. At this age 

 a transverse constriction (r) is present in the dorsal region of the 

 primary fore-brain, just dorsal to the optic vesicle. This is a per- 

 manent constriction that ultimately divides the primary fore-brain 

 into the prosencephalon and thalamencephalon. 



The 5 segments of the medulla (7 — 77), are more sharply 

 defined than in younger specimens. The auditory vesicle occupies an 

 intersegmental position between segments 10 and 11 (Fig. 32 au.vs). 

 Up to this time the segments of the hind-brain have been equal in 

 size and alike in form. As observed in Fig. 33, segment 9, which is 

 connected with the 7th and 8th cranial nerves, has now become 

 wedge-shaped with the apex turned dorsad and ever after this af- 

 fords an anatomical land mark. The two adjacent segments are 

 modified in a reciprocal manner. This condition prevails till the seg- 

 ments ultimately disappear. 



Figs. 34 and 35 represent ventral and profile views of the en- 



