304 JOHN WARKEN 



Posterior commissure 



The posterior commissure is constant in all vertebrates and 

 always precedes the superior commissure except in Ammocoetes. 

 This tract is usually regarded morphologically as a part of the 

 mid brain and as the boundary between mesencephalon and dien- 

 cephalon. However, if we regard the 'Schalthirn' or synenceph- 

 alon as a part of the fore brain as it really is, a portion of the com- 

 missure must be assigned to the diencephalon. The first appear- 

 ance of the commissure in the lizard is seen in fig. 8, Lacerta 

 muralis, 4.5 mm. It forms a well marked flat band of fibers in 

 the outer part of the wall of the brain and is confined entirely to 

 the synencephalon. This is still the case in fig. 9, Lacerta mu- 

 ralis, 5 mm. Kupffer (57), fig. 286, states that the posterior com- 

 missure always appears first in the posterior part of the synen-. 

 cephalon. At first there is a section of the synencephalon between 

 this commissure and the epiphysis where there are no commissural 

 fibers, but later in most cases this part is invaded by fibers and 

 absorbed by the commissure. This is not the case, however, in 

 Necturus where there is a portion of the synencephalon which 

 always remains unoccupied by commissural fibers. After this 

 stage it grows backward into the mid brain and there it spreads 

 out and blends with the outer layer of the wall. It becomes very 

 diffuse and has no definite caudal limits. In later stages as the 

 mid brain encroaches on the fore brain the commissure becomes 

 doubled over on itself and an anterior and posterior part is formed 

 separated by a sort of septum which really consists of the outer 

 cells of the brain wall. These points have been emphasized by 

 Terry (102) in Batrachus. This encroachment of the mid brain 

 and the doubling up of the commissure compresses the roof of 

 the synencephalon to such a degree that this segment of the fore 

 brain becomes virtually suppressed. It is a question whether 

 we have to deal here with two commissures, or two distinct parts 

 of one commissure, one forming first in the roof of the synen- 

 cephalon and a second of later appearance joining this and spread- 

 ing into the mid brain. I have not been able as yet to determine 

 this point. 



