180 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



with the axis of the midbrain vesicle, is shown in Figure 49, Plate 7. 

 The vesicle of the primary forebraiu (I) appears as an almost circular 

 enlargement of the anterior portion of the neural tube. Behind this, 

 and separated from it by a constriction, lies the narrower and somewhat 

 more elongated midbrain vesicle (II). Posteriorly a small portion of the 

 "Hinterhirn " vesicle also appears in the section. 



Sagittal sections of embryos at this stage are seen in Figures 8 and 9, 

 Plate 3. Faint dorsal constrictions separate forebraiu, midbrain, and 

 " Hinterhirn " (III), the separation between midbrain and " Hinterhirn " 

 being very slight. A deep depression in the floor of the forebraiu marks 

 the position of the infundibulum, which is bounded posteriorly by a faint 

 constriction, the first indication of the tuberculum posterius (Kupffer). 

 Another constriction of the ventral wall of the neural tube is seen behind 

 the tuberculum posterius in the region of the midbrain, — the plica en- 

 cephali ventralis. In later stages the I'egion of this constriction becomes 

 the point of greatest flexure of the neural tube. The constrictions 

 marking off the brain vesicles appear as rather broad depressions, not 

 sharply defined as are the constrictions between neuromeres. The brain 

 vesicles are also seen to be considerably larger than the hindbrain 

 neuromeres, the diff'erence in size constantly increasing from this stage 

 onwards. Except for a local thickening of the lateral zones, the two 

 anterior brain vesicles are structurally quite comparable with the hind- 

 brain neuromeres. They similarly involve all three zones of the neural 

 tube. 



An examination of embryos at a stage with 28 to 30 somites, i.e. 

 early in Balfour's stage H, shows that slight changes have occurred. A 

 parasagittal section of such an embryo is shown in Figure 46, Plate 7. 

 The anterior vesicle, the forebraiu, is so cut that one sees its lumen. 

 Behind this, and separated from it by a constriction which extends to 

 the ventral portion of the tube, lies the midbrain, which dorsally is a 

 single expansion passing almost without constriction into the hindbrain. 

 The depth of the constriction is much less than it appears to be in this 

 figure, because the section passes to one side of the median plane. In 

 the ventral half of the midbrain there is a constriction, which more 

 median sections of this stage (Fig. 13, Plate 3) show to correspond with 

 the region of sharpest flexure of the neural tube (plica encephali ven- 

 tralis). This constriction does not extend, however, to the dorsal portion 

 of the neural tube, and therefore is not equivalent to a constriction 

 which separates neuromeres. By it the midbrain is separated ven- 

 trally into two lateral expansions on each side of the head, — one 



