418 CHARLES HILL, 



(Figs. 25 — 27), this evagination is present in the dorsal region of the 

 neural tube and involves the primary segments 1, 2 and 3. In the 

 profile view (Fig. 26), the second segmental groove that forms the 

 dividing line between segments 2 and 3 is restricted to the base of 

 the brain and to the ventral portion of the optic evagination. The 

 latter in this view appears partly divided. In Fig. 23 this groove 

 completely divides the optic evagination; in embryos a little older 

 (Fig. 26), the groove is shortened and affects only the base of the 

 optic expansion, while in still older embryos (Figs. 29, 31), this groove 

 has entirely disappeared. 



In embryos with 5 to 7 somites and in favorable specimens, the 

 dorsal portion of segments 4 — 7 shows a lateral expansion just below 

 the neural ridge (Fig. 26). In the divided embryos of this age these 

 expansions may be detected as faint depressions on the inner aspect 

 of the neural tube (Figs. 24 and 27). Locr, '97, has described these 

 structures as "accessory optic vesicles" and has observed them in 

 favorable undissected embryos of this age and also in sections. The 

 fact that these transient "vesicles" appear as a direct caudad con- 

 tinuation of the optic expansion which itself is primarily divided (see 

 Figs. 23 and 26), coupled with the well known fact that the optic 

 evagination extends at first along the whole length of the primary 

 fore-brain and later retains a union with only the distal portion of 

 this brain, are collectively suggestive of a segmental and multiple 

 origin of the visual organ in the Vertebrate phylum. 



In Figs. 28 and 29, that represent two views of the encephalon 

 of a dissected embryo with 7 somites, 26 hours old, the 3 anterior 

 segments are about to fuse to form one large division, the primary 

 fore-brain. The second transverse groove is confined exclusively to 

 the base of the brain without encroaching upon the ventral portion 

 of the optic evagination as in Fig. 26. The "accessory optic vesicles" 

 have disappeared. In the dorsal view three transverse segmental 

 grooves (c, e and g) are relatively deep. 



The portion between the grooves c and e represents the mid- 

 brain and includes segments 4 and 5. That portion between the 

 grooves e and g includes segments 6 and 7. As will appear later, 

 segment 6 develops into the cerebellum and 7 is the anterior seg- 

 ment of the medulla. We have here a lateral neural expansion, 

 analogous to that observed in the trout (page 407, Figs. 2, 3), which 

 also involved segments 6 and 7. At this age therefore, the distal 



