414 CHARLES HILL, 



very faint. Segments 4 and 5 are present as described in Fig. 13. 

 Segment 5 is relatively small and appears compressed between seg- 

 ments 4 and 6. Posterior to the deep constriction (/") the 5 segments 

 of the medulla (7 — U) are regular and equal in size, and confined to 

 the ventral region of the encephalon. 



In embryos 36 days old (Fig. 15) the segments of the medulla 

 are about to disappear. With the lateral expansion of the neural 

 tube they seem to have passed ventrad and are now confined to the 

 floor of the medulla which is covered by a high unsegmented brain- 

 roof. The transverse groove between segments 3 and 4 is absent, and 

 only the posterior commissure in the dorsal region marks the posterior 

 limit of the primary fore-brain. Two or three folds have appeared 

 in the lateral walls of the large infundibulum. These in sections are 

 found to aifect both the internal and external surfaces and are thus 

 readily confused with true segmental grooves, but must be given 

 ditferent morphological value. Of the 2 segments that constitute 

 the mid-brain only the posterior one (segment 5) remains distinct, 

 the anterior one (segment 4) having fused with the posterior segment 

 of the fore-brain. Segment 6, the cerebellum, has become very broad 

 and as in younger specimens, extends to the dorsal surface of the 

 encephalon. 



The preceding description justified the following conclusions: 



1) The encephalon of Salmo is divided into 11 primary segments 

 or joints. 



2) The dividing planes between these segments are marked by 

 transverse external an corresponding internal constrictions or 

 grooves. 



3) These segments appear very early and antedate the historical 

 divisions known as fore-brain, mid-brain and hind-brain, 



4) The latter divisions are not simple, but compound and include 

 a definite number of primary segments as follows: 



The fore-brain includes 3 segments. 



The mid-brain includes 2 segments. 



The hind brain includes 6 segments. 

 The anterior segment of the hind-brain represents the cerebel- 

 lum, the other 5 constitute the medulla. 



B. Encephalic Segments of the Chick. 



In order to secure a very close series of embryonic stages, up- 

 wards of 700 eggs were incubated and opened between the ages of 



