410 



CHARLES HILL, 



Fig. D. 



■yc com. 



the encephalic expansion that was mentioned in the description of 

 Fig. 5. 



From an external view 5 segments can still be detected in that 

 portion of the brain that lies cephalad to the cerebellum {1—5 Fig. 8). 

 The anterior segment in profile view appears somewhat elliptical. 

 Segment 2 has become very narrow dorsally while ventrally it ex- 

 pands into the relatively large infundibulum, which is directed caudad 

 and extends back to a point opposite the middle portion of the mid- 

 brain. Segment 3 is dorsally very broad while ventrally it tapers to 

 a point. Externally the mid-brain now appears unsegmented, save for 

 a ventral constriction that lies just dorsal to the posterior margin of 

 the infundibulum. 



Sketch D is a camera 

 tracing of the cephalic end 

 of a living embryo, 32 days 

 old, rotated towards the 

 right and viewed from a 

 point slightly in front and 

 above the embryo. The 

 sketch was made to show 

 the position of the posterior 

 commissure {2^0. com) which 

 at this age is present as an 

 opaque transverse band of 

 fibres that pass across the 

 roof of the encephalon and 

 unite on each side with the anterior portion of segment 4 and the 

 posterior portion of segment 3. Simultaneously with the formation 

 of the commissure, a groove or notch appears in the same position 

 in the dorsal wall of the brain. This groove has thus a difierent 

 history from those that are present in the base of the brain and 

 therefore a correspondingly different morphological value. 



The oldest living embryo in which I have been able to satis- 

 factorily observe the presence of neural segments is represented by 

 Fig. 9. The segments of the medulla are fainter than in earlier stages 

 but it should be noted that they can be traced in the fore- and mid- 

 brain regions as well as in the medulla. In front of the cerebellum 

 the 2 segments of the mid-brain {4 and 5) can be seen on the inner 

 aspect of the encephalon through the transparent brain wall. In like 

 manner the 3 segments of the primary fore-brain appear {1 — 5). 



