The Drain of Diemydylus Viridcscens 283 



nence. With regard to the position of these commissures; 

 the lamprey brain is in closer relationship with the anurous 

 than the urodele or fish type of brain, though, as was sug- 

 gested by Wilder (52) from other considerations, the brain of 

 lamprey is much like that ofurodeles. It is hoped that the 

 facts now known with regard to these commissures may be 

 brought into harmony by further embryological study. 



CRISTA. 



In diemyctylus an object which, in comparison to the size 

 of tlie brain, is large, projects on the meson, freely into the 

 cavity of the aula, as shown both in transections and in 

 frontal sections. In the section cephalad of the portae (Fig. 

 16), it is seen that the endyma is reflected over a rounded 

 surface, the crista, to form the last remnant of the partition be- 

 tween the hemicerebrums. In frontal sections its base (Fig. 45) 

 joins the floor of the aula, but then projects into the aula and 

 toward the cerebral commissures. It contains white fibers, 

 apparently non-nervous, with a herring-bone arrangement on 

 either side of a loop of pia which extends far into it (Fig. 46). 

 Its dorsal side (Fig. 47) showing endymal cells in face view, 

 retreats to the general level of the terma, except that a slight 

 ridge extends dorsad from it (Fig. 37), and this ridge (Fig. 

 36) is the point from which the endyma is reflected to form 

 the auliplexus and (Fig. 35) the mesal walls of the cerebrum. 



In the larva (Fig. 71, 72) a trace of this structure is visi- 

 ble. Here, owing to the large aula, it is relatively farther 

 cephalad. In the twelve-day embryos it is far cephalad, be- 

 ing the first indication of the restriction of the aula. In later 

 embryos it still is far cephalad (Fig. 73, 74). In all these 

 cases this projection bears the same relation to the rudi- 

 mentary mesal walls of the cerebrum as in the adult, though, 

 in the embryos, the auliplexus is not yet developed. Hence, 

 morphologically it is the dorsal limit of the terma, a kind of 

 fixed point beyond which the olfactory lobes and the cere- 

 brum project in their growth and from which the auliplexus 

 is reflected. 



In the amia a structure which agrees in position with the 



