neal: nervous system in squalus acanthias. 181 



antei'ior, the other posterior, — while dorsally it remains a single dilata- 

 tion. The anterior of the two expansions narrows as it extends ventrally, 

 and terminates at a point in the ventral wall near, but anterior to, the 

 tuberculum posterius. The posterior of the two midbrain expansions is 

 bounded behind by the faint lateral constriction between midbrain and 

 hindbrain vesicles. 



The conditions shown in a sagittal section at this stage are not essen- 

 tially different from those presented at the stage previously described 

 (Fig. 13, Plate 3). The forebrain, midbrain, and "Hinterhirn" vesi- 

 cles are separated by very faint dorsal constrictions. In the constric- 

 tion between forebrain and midbrain vesicles appears later Miss Piatt's 

 *' thalamic nerve." Ventrally two constrictions are seen, one corre- 

 sponding with the tuberculum posterius, and the other, more posterior, 

 with the point of greatest flexure of the neural tube. Two frontal sec- 

 tions of an embryo at this stage are shown in Plate 7, Figures 48 and 

 50. Figure 48 represents the more dorsal of the two, and shows only 

 the expansion of forebrain and midbrain vesicles separated by the pri- 

 mary constrictions spoken of above. A small portion of the " Hinter- 

 hirn" is shown. The section shown in Figure 50 is more ventral, being 

 in a plane about midway between the dorsal and ventral sides of the 

 neural tube. An arrow is drawn at the constriction separating forebrain 

 and midbrain vesicles. This constriction corresponds with the one seen 

 in the more dorsal section, also indicated by an arrow. Behind this, in 

 the region of the midbrain, another constriction appears, one which was 

 not seen in the dorsal section. This may be traced in more ventral 

 sections into the constriction previously described as occupying the floor 

 of the midbrain at a point corresponding with the point of greatest flex- 

 ure of the neural tube. In my opinion everything in front of the arrow 

 belongs to the primary forebrain, the lateral walls of which are expanded 

 to form the optic vesicles. Behind these two vesicles are seen " two well 

 marked constrictions and two convolutions" of the neural wall which 

 show radially arranged nuclei.^ It is found in later stages that the pos- 

 terior of the two constrictions corresponds in position with the posterior 

 commissure, and therefore that what lies anterior to this constriction must 

 be considered as part of the thalamencephalon. It is seen, therefore, 

 that the constriction between primary forebrain and midbrain vesicles 

 does not correspond with the posterior commissure, which in later stages 

 forms by common consent the anterior boundary of the midbrain (see 



1 Probably the two " neuromeres " of the thalamencephalon described by Orr 

 ('87). 



