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groove (c) marks the position of the posterior commissure and there- 
fore the anterior limit of the mid-brain. Segment 1 is elliptical in 
this view and represents the morphological front of the brain. The 
olfactory nerve is connected with this segment. Segment 2 is narrow 
in the dorsal region and expands in a ventrocaudad direction to pro- 
duce the infundibulum. This segment undergoes a greater change than 
any of the other cephalic segments. The optic nerve is connected 
with this joint (Fig. 8 op. n.). Segment 3 tapers ventrally. The 
posterior commissure develops along its posterior border and the two 
epiphyses evaginate from its dorsal unsegmented roof. Segments 4 
aud 5 represent the mid-brain and bear a closer resemblance to the 
joints of the medulla than do the three anterior segments that re- 
present the fore-brain. 
Fig. 10. Salmo purpuratus. Left half of the anterior portion of a divided 
embryo with 54 somites, 36 days old. Internal encephalic surface exposed to view. 
Letters and figures the same as in Fig. 8. >< 60 diameters. 
I have found 11 encephalic segments in the Trout embryo both 
in the living, in the dissected and in the divided specimens. These 
segments antedate the formation of fore-, mid- and hind-brain vesicles, 
The latter are, therefore, compound structures and not homologous 
parts, for the fore-brain embraces three segments (1, 2, 3), the mid- 
brain two (4 and 5), and the hind-brain six (6—11). 
Neural Segments in the Chick. 
I have traced the neural segments in the Chick from embryos 
with one somite, 21 hours old, through a close series of specimens 
with open, closing and closed neural grooves, up to embryos in the 
