Developmental history of primary segments of the vertebrate head. 413 



and in front, above the 3. segment of the fore-brain. From my 

 observations on living embryos and from a study of sections, it 

 appears that this furrow is produced by a reduction of cellular 

 elements on the inner surface of the brain wall of this region. This 

 reduction spreads dorsally so as to form two diagonal furrows (Fig. 12). 

 In later stages the inner wall of the brain-roof of this region presents 

 a ragged appearance (Fig. 13), while in still older embryos the sur- 

 face becomes more even (Fig. 14) and ultimately smooth (Fig. 15). 

 During this process the outer surface is unaffected while the brain- 

 roof becomes very thin. The method of their appearance and their 

 history indicate that these furrows are not of segmental value. 



If we now return to the segmental condition of embryos 24 days 

 old (Fig. 12) as observed in divided embryos, we find in this stage, 

 the 5 anterior segments (1 — 5) confined to the base and the lateral 

 walls of the encephalon. In this respect they resemble the segments 

 of the medulla (7 — 11). The 3 segments of the primary fore-brain 

 are present as described in Fig. 11. The groove that separates seg- 

 ment 3 and 4 and forms the posterior limit of the primary fore-brain 

 has become very indistinct. On the other hand the constriction that 

 separates the 2 segments of the mid-brain {4 and 5) is deep. The 

 5 anterior encephalic segments can thus be counted and are similar 

 both in position and form to the 5 segments (7—11) of the medulla. 



Fig. 13 shows the inner surface of the encephalon of a divided 

 embryo 26 days old. Segment 2 is more compressed in the dorsal 

 region than in younger embryos, while its ventral expansion, that 

 forms the infundibulum , is much enlarged. In the dorsal region 

 segment 3 is broad and presents a depression which at this age gives 

 the segment when sectioned, the appearance of being double (Fig. 43). 

 The 2 segments of the mid-brain (4 and 5) and segment 6, the 

 cerebellum, are very distinct. The latter alone extends to the dorsal 

 surface of the brain. The depression (f) remains conspicuous as ob- 

 served in other specimens. From the bottom of this depression several 

 secondary grooves radiate into the base of the brain. From their 

 irregularity and their inconstancy I have assigned to them no morpho- 

 logical value, although in sections they often present the appearance 

 of true segmental limits, and are therefore very confusing. 



In embryos 2 days older, segment 2 tapers nearly to a point 

 in the dorsal region while ventrally it is very broad and expands to 

 form the relatively broad infundibulum. Segment 3 is still present 

 and distinctly wedged-shaped although its posterior border has become 



