Developmental history of primary segments of the vertebrate head. 421 



oM. The segmental grooves of the medulla at this age terminate 

 dorsally in a longitudinal lateral ridge, along which the thin unseg- 

 niented roof of the 4th ventricle it attached. The cerebellum (seg- 

 ment 6), forms the anterior expanded wall of the 4th ventricle just 

 as in trout embryos 30 days old. The thalamencephalon is divided 

 into two portions by a dorsal constriction (s) that made its appearance 

 during the third day of incubation. They very late appearance 

 of these divisions, and the fact that they are produced by a dorsal 

 constriction render it highly improbable that they have the same 

 morphological value as the early encephalic segments. 



From this description it appears that the encephalic segments of 

 the chick and the trout differ only in one detail. In the trout the 

 dividing lines between the segments form grooves on both the external 

 and the internal surfaces. In the chick after the neural groove has 

 closed to form a tube, the dividing lines on the inner surfaces of the 

 medulla are represented by transverse ridges. It is to be remembered, 

 however, that each ridge is preceded by a transverse groove, and that 

 many of the ridges have at their apices traces of this groove, as the 

 description of sections will show. 



With this exception the conclusions reached from the study of 

 the encephalic segments of the chick are identical with those reached 

 from a study of the trout (see page 414). 



C. Study of Sections, Chick and Trout. 



The value of making dissections even of the smallest embryos 

 can scarcely be overestimated. The segmental furrows can be identi- 

 fied with greater certainty on the dissected embryos than by means 

 of sections. The surface observations, also, serve as a check upon 

 the sections and vice versa. As my series of sections agree com- 

 pletely with the surface studies, the following description will be brief 

 and limited to the most typical sections. 



Fig. 42, Plate 30, represents a parasagittal section through the 

 cephalic region of a trout embryo 22 days old, and should be com- 

 pared with the divided encephalon of the same age (Fig. 11, Plate 1). 

 11 encephalic segments are present (/ — li). These are not only 

 separated from each other by dorsal and ventral constrictions but by 

 the transverse narrow septa, which, as Orr has stated, may be nothing 

 else than parts of cell walls "which are made conspicuous by lying 

 in a straight line". The feature of chief interest is 5 segments in 

 fore- and mid-brain, similar to those in the medulla. Modification of 



