Developmental history of primary segments of the vertebrate head. 417 



ments can be detected only along the thick dorsal margin (Fig. 25 

 1 — 3). In the ventral zone these 3 segments appear better defined 

 (see Fig. 26 l—S). The remaining 8 segments may be observed 

 in either a dorsal or lateral view. The transverse grooves that mark 

 their anterior and their posterior limits, pass around the neural axis, 

 and in the posterior encephalic region where the neural tube is still 

 open, they become directly continuous with segmental lines that are 

 present on the inner surface of the encephalou, as will appear later 

 from a study of the divided specimens. The developmental history 

 shows that, after the neural groove has closed to form a tube, these 

 identical posterior segments persist and can be recognized as neuro- 

 meres of the medulla, so extensively described by other authors in all 

 Vertebrate groups. 



In these figures the fore-, mid- and hind-brain divisions are better 

 seen than in the earlier stages but it should be remembered that they 

 are not simple but compound — as this shows clearly in a side view 

 of the encephalon (Fig. 26). The fore-brain has 3 primary seg- 

 ments {1 — 5); the mid-brain has 2 (-5 — 4), and the hind-brain has 

 6 (5 — 11). Fig. 27 represents the encephalon just described after 

 it has been divided and the inner surface of the left half exposed. 

 There are, as in Fig. 24, 11 segments in the thickened dorsal 

 margin (l—ll). Along this margin the dividing lines between these 

 segments appear as transverse grooves, while in the lateral and ven- 

 tral region they are continued as transverse ridges or crests. It should 

 be remembered that all these ridges are preceded by transverse 

 grooves. We can thus detect on the inner surface and along the 

 dorsal margin the same number of primary segments as were ob- 

 served on the external surface. 



We have here the first traces of the typical "neuromeres" de- 

 scribed by Okr, '87, as follows: "Each neuromere is separated from 

 its neighbor by an external dorso-ventral constriction, and opposite 

 this an internal sharp dorso-ventral ridge." In the description of 

 sections of these segments it will appear that grooves are present at 

 the apices of many of these ridges, and that the ridges are probably 

 produced passively by an intrasegmental lateral expansion of the 

 neural tube. 



The optic evagination bears such a close relation to the 3 

 anterior neural segments that this description would be incomplete 

 without its developmental history. This evagination is present in 

 embryos with open neural grooves. In the three figures just described 



