416 



CHARLES HILL, 



thickened dorsal crest the primary segments are very conspicuous and 

 correspond in number and position to the segments described on the 

 external surface, 2) in the ventral and lateral grooves the dividing 

 lines between segments 3 and 4, and segments 5 and 6, are trans- 

 verse ridges (& and e'), while farther caudad the dividing lines are 

 grooves. The transverse ridge & thus marks the posterior border of 

 the fore-brain, which, as stated above, includes segments 1—3; that 

 portion between the ridges & and e' represents the mid-brain and in- 

 cludes the primary segments 4 and 5. It thus appears that the joints 

 or segments on the inner surface of the neural groove correspond in 

 number and position to those observed on the external surface. 



Fig. 21 represents the left half of the divided encephalou of an 

 embryo with 3 somites 23 hours old, one hour younger than the 

 embryo just described. The inner surface is exposed to view, showing 

 11 transverse grooves corresponding to those described in the 

 other specimens, and agreeing in number and position to those on the 

 external surface. From a comparison of Fig. 21 and Fig. 24, it 

 appears that the transverse ridges c' and e' (Fig. 24), are preceded 

 by transverse grooves. This is also true of the segmental ridges that 

 are present on the inner surface of the hind-brain of older embryos. 



I have traced these segments, by dissection through a close 

 series of younger specimens to embryos with one somite (Plate 29, 

 Fig. 16—20). The 11 constrictions are present on both inner and 

 outer surfaces of the open neural groove, and are not only continuous 

 with each other, but appear to fall in the same transverse planes. 

 It should be noted that these segments are constant in number and 

 nearly equal in size, and that they appear earlier in the ontogeny than 

 the historic encephalic divisions, fore-brain, mid-brain and hind-brain. 

 The latter are not strictly segmental divisions and are differently 

 composed, the fore-brain having 3 segments, the mid-brain 2, and the 

 hind-brain 6. 



b) Embryos with 6—7 Somites (Ages 25 — 26 hours). 



Figs. 25, 26 and 27 represent different views of the encephalon 

 of an embryo with 6 '/a somites, about 25^/2 hours old. In the 

 dorsal view. Fig. 25, it will be seen that the neural groove has 

 closed in the region of segments 3 to 7 but still remains open both 

 caudad and cephalad. In the dorsal and lateral views, 11 encephalic 

 segments can be counted. At this stage the evagination of the lateral 

 eyes is so advanced that in the dorsal view the 3 anterior seg- 



