Developmental history of primaj-y segments of the vertebrate head. 411 



These 5 segments of the anterior cephalic region are now confined 

 to the floor and sides of the brain in a manner very similar to the 

 5 segments of the medulla. In both regions the segments at this 

 age are covered by an unsegmented, thin and very transparent roof. 

 Closely applied against the lateral wall of segment 1 is the 

 olfactory pit {olf.pt). As in other specimens, the dorsal portion of 

 segment 2 is very narrow. The transverse dorsal depression formed 

 by the fibres of the posterior commissure (po.com) has become deeper 

 than in earlier stages (sketch D). Just cephalad to this depression 

 near the median plane, are the two evagiuations that form the right 

 and left epiphysial vesicles whose history I have traced in former 

 contributions (Hill, '91 and '94). 



b) Neural Segments of Salmo observed in Divided 



Embryos. 



The neural segments previously described are even more favorably 

 seen by observing embryos that have been divided into lateral halves. 

 The inner surfaces of the brain-walls are thus exposed entirely free 

 from any cell layers. This form of manipulation can be used only 

 after the neural cavity is fully formed (embryos about 20 days old, 

 31 somites). Between 20 and 36 day — stages I have a very complete 

 series of divided embryos and have been able to trace in them un- 

 broken continuity of the segments, but I shall limit the description 

 to a few typical stages. 



Fig. 10 represents the inner surface of the right cephalic half of 

 a divided embryo 20 days old with 31 somites. Eleven transverse 

 grooves are present that divide this surface into 11 segments (1 — 11). 

 Fig. 10 compared with Fig. 1, representing the external encephalic 

 surface of a dissected embryo one day younger, shows that the ex- 

 ternal grooves of the latter correspond with internal grooves of the 

 former and thus divide the neural axis into comparable segments. 



As represented in this figure (Fig. 10) the first 5 segments, 

 that form the primary fore-brain and hind-brain, are more distinct 

 when viewed in the divided embryo than when studied in the dis- 

 sected or the living specimens. In this view the anterior segment 

 is biconvex, the greater curvature forming the front of the neural 

 axis. Segment 2 is dorsally narrow while ventrally it is broad and slightly 

 expanded in a caudad direction to form the first traces of the in- 

 fundibulum (inf). Towards the ventral portion of this segment there 

 is present a small pit or depression (op.u). This is the remnant of 



