360 _ 
Fig. 6. Fig. 7. 
Fig. 6. Dorsal view of the same encephalon that is represented in Fig. 5. 
Fig. 7. Salmo purpuratus. Encephalon of a dissected embryo with 34 somites, 
22 days old. Left surface view. op. n. optic stalk. Other letters and figures the same 
as in Fig. 5. > 60 diameters. 
terior segments that represent the fore- and mid-brains, are well 
shown. They are separated by external grooves that pass around 
the brain similar to those of the medulla. Segment 1 is elliptical, 
segment 2 is distinctly wedged-shape and expands ventrally to form 
the infundibulum. 
It will be observed that the optic stalk is connected with this 
segment (Fig. 7 op. n.). Segment 3 is also wedged-shaped, with the 
base upwards instead of downwards as in segment 2. The posterior 
commissure develops at the point (c) and the three anterior segments 
represent, therefore, the primary fore-brain. Segments 4 and 5 form 
the mid-brain and segment 6 represents the cerebellum. All these 
segments can be distinctly counted. They antedate the formation of 
the larger divisions of fore-, mid- and hind-brains and have the same 
developmental history as the neural segments of the medulla. 
Neural Segments of Salmo. Divided Embryos. 
The neural segments previously described are more favorably, 
shown in embryos that have been divided into lateral halves. The 
inner surface of the brain-walls is thus exposed entirely free from any 
cell layers. This kind of manipulation can be used only after the 
