Developmental history of primary segments of the vertebrate head. 445 



Plate 30. 



Fig. 36. Encephalon of a chick embryo, 39 hours old. Dorsal 

 view. X ^0 diameters. Dividing line between segments 6 and 7 has 

 now become very distinct. 



Fig. 37. Encephalon of chick embryo, 43 hours old. Right profile 

 view. X 50 diameters. The 6 segments of the hind-brain are sharply 

 defined. Segment 6 represents the medulla. Segment 9 is wedge- 

 shaped. The 4th ventricle has appeared, covered by a thin roof. 



Fig. 38. Dorsal view of the same encephalon as represented by 

 Fig. 37. X 50 diameters. A dorsal constriction (r) has appeared in 

 the primary fore-brain, dividing this region into prosencephalon and 

 thalamencephalon. 



Fig. 39. Right profile view of the encephalon of a chick embryo, 

 47 hours old. X 50 diameters. Limits of the neural segments in the 

 medulla are sharply defined. The dorsal constriction (r) divides the 

 primary fore-brain as described in Fig. 38. 



Fig. 40. Encephalon of a chick embryo, 50 hours old. Right 

 profile view. X ^^ diameters. A dorsal groove (s) has appeared, 

 dividing the thalamencephalon. These large anterior divisions are very 

 different, however, from the neural segments of the medulla {6 — 11). 

 Failure to recognize this has led to confusion. 



Fig. 41. Encephalon of a chick embryo, 80 hours old. Right 

 surface view. X ^^ diameters. The segments of the medulla are 

 about to disappear. Segment 6 represents the cerebellum and forms 

 the anterior boundary of the 4th ventricle. The portion between c and 

 r represents the thalamencephalon and is divided by the groove s as 

 represented in Fig. 40. The portion in front of r represents the 

 prosencephalon. A median furrow in the dorsal region has divided this 

 into the right and the left lobes of the cerebrum. 



Fig. 42. Parasagittal section through the anterior portion of 

 Salmo purpurafus, 22 days old. X ^^ diameters. Segment 6 repre- 

 sents the cerebellum. Five segments (1 — 5) can be counted in front 

 of this. Segments 1, 2, 3 represent the fore-brain, and segments 4 and 

 5 represent the mid-brain. 



Fig. 43. Horizontal section through the head of S. purpuratus, 

 26 days old. X 60 diameters. The section divides the right eye, but 

 i:)asses dorsal to the left eye. It passes through the dorsal encephalic 

 region, therefore segment 2 is narrow and segment 3 is broad. Com- 

 pare this with Fig. 13, Plate 1. 



Fig. 44. Horizontal section through the anterior portion of a chick 

 embryo with 4 somites, 23^/2 hours old, with open neural groove. 

 11 neural segments are present (1 — 11). The dividing lines between 

 these segments appear as external and internal transverse grooves. Com- 

 pare this with Figs. 19 and 20 representing an embryo of the same 

 age as Fig. 44. 



Fig. 45. Horizontal section through the right wall of the medulla 

 of a chick embryo with 14 somites, 33 hours old. X '^"^^ diameters. 



