412 CHARLES HILL, 



the lumen that primarily communicated with the hollow optic stalk. 

 Segment 3 is dorsally broad and tapers ventrally. Its posterior limit 

 in the dorsal region is marked by a depression (c) which in older 

 stages marks the position of the posterior commissure and therefore 

 forms the posterior limit of the primary fore-brain. Between this 

 depression (c) and the deep constriction {f) which, as previously ob- 

 served, forms the anterior limit of the medulla, 3 segments are 

 present (■^, 5 and 6). One of these, segment 6, is the cerebellum 

 and the other two, segments 4 and 5, represent the mid-brain. The 

 transverse grooves that divide the anterior encephalic region into 

 segments are distinct and can be counted with as much accuracy as 

 the transverse grooves that divide the medulla into corresponding 

 sections or joints. In both regions the segmental grooves are better 

 defined on the internal than the external brain surface. 



In divided embryos 2 days older, a very considerable change 

 has taken place on the inner aspect of the encephalon. This change 

 is represented in Fig. 11 which should be comparad with Fig. 3. 

 The latter represents the external surface of the same age. In the 

 divided specimen (Fig. 11) the deep constriction (/), that forms the 

 anterior limit of the medulla, is at once very apparent. The region 

 in front of this constriction constitutes, therefore, the primary fore- 

 brain, mid-brain and cerebellum and it is a great satisfaction to have 

 the segmental joints so distinctly marked in the controverted territory 

 of fore- and mid-brain. Segment 1 is elliptical and is but little 

 changed, either in form or position from the description given of it 

 in Fig. 10. Segment 2, not only tapers dorsally but has expanded 

 ventrally in a caudad direction to form the relatively prominent in- 

 fundibulum (inf). Segment 3 tapers ventrally and. as in Fig. 10, has 

 at its posterior dorsal margin a depression (c) which, as stated before, 

 marks the posterior limit of the primary fore-brain. A horizontal 

 furrow intercepts the 2 segments of the mid-brain giving them thus 

 a dorso-ventral length equal approximately to the length- of the seg- 

 ments of the medulla, and like the latter, have become covered by an 

 unsegmented brain-roof. The developmental history of this horizontal 

 furrow clearly shows that it has no segmental value. Because of its 

 prominence in older embryos it seems desirable in this place to trace 

 this history. 



In divided embryos 24 days old (Fig. 12) the horizontal furrow, 

 mentioned above, is deeper and longer, and forms not only the dorsal 

 limit of the 2 mid-brain segments but extends over the cerebellum 



