442 CHARLES HILL, 



Fig. 3. Encephalon of a dissected embryo with 34 somites, 22 

 days old. Left surface view. The deep groove f forms the anterior 

 limit of the medulla. The groove c marks the anterior limit of the 

 mid-brain. The 3 segments in front of this are very distinct, and 

 constitute the fore-brain. The optic nerve {opt) is attached to the 

 ventral portion of segment 2. 



Fig. 4. Anterior portion of a living embryo with 33 somites, 



22 days old. Left profile view. Segment 6, the cerebellum, and seg- 

 ment 5, the anterior segment of the mid-brain, show very distinctly. 



Fig. 5. Anterior portion of a living embryo with 35 somites, 



23 days old. Dorsal view. This figure is sketched from two points 

 of view as indicated by the »»-* in Fig. 6. The eleven encephalic segments 

 (i — IT) are separated by transverse grooves that show very distinctly. 



Fig. 6. Left profile view of the same embryo that is represented 

 in Fig. 5. The dorsal groove (f) has become very distinct. This 

 groove marks the anterior limit of the medulla. 



Fig. 7. Anterior portion of a living embryo with 40 somites, 

 26 days old. Dorsal view, sketched from one point of view. Since the 

 embryos curve ventrally to conform to the spheroid yolk the medulla, 

 in this figure, appears fore-shortened. The 3 segments {1, 5, 3) of 

 the fore- brain are very marked on the inner encephalic surface. On 

 this surface the limits of the 2 segments that form the mid-brain 

 (segments 4 and 5) are also distinct. 



Fig. 8. Encephalon of a dissected embryo with 42 somites, 27 days 

 old. Left surface view. The groove c marks the anterior limit of the 

 mid-brain. The external surface appears unsegmented. The 3 seg- 

 ments of the fore-brain (segs. 1, 2 and 3) can still be counted. Segment 

 2 has become more wedge-shaped as compared with younger stages. 



Fig. 9. Salmo fario. Anterior portion of a living embryo with 

 55 somites, 38 days old. Right profile view. The 5 segments of 

 the fore- and mid-brains (segs. 1 — 5) show on the inner surface of the 

 brain through the transparent brain wall. 



Fig. 10. Right half of the anterior portion of a divided embryo 

 with 31 somites, 20 days old. The internal surface of the encephalon 

 is exposed to view. The groove c marks the anterior limit of the mid- 

 brain. The 3 segments of the fore-brain (segs. 1, 2 and 3) affect 

 the ventral as well as the dorsal zone. The optic stalk (op.n) is con- 

 nected with segment 2. Compare this figure with Fig. 1, external sur- 

 face of an encephalon one day younger. 



Fig. 11. Right half of the anterior portion of a divided embryo 

 with 33 somites, 22 days old. Internal surface of encephalon exposed 

 to view. The groove f marks the anterior limit of the medulla. All 

 6 segments in front of this groove can be counted. Segment 2 

 expands to form the infundibulum. 



Fig. 12. Left half of the anterior portion of a divided embryo 

 with 37 somites, 24 days old. Internal surface of encephalon exposed 

 to view. Compare this figure with Fig. 8, that shows the external 

 surface of an encephalon 2 days older. 



