The Brain of Diemydyhis Viridescens 287 



upon it, and in the adult it is a very irregular sac b'ing in 

 the midst of the supraplexus (Fig. 51, 53, 59, 60). The open- 

 ing is upon the meson at a point between the blood vessels 

 supplying the dia- and auliplexuses. (See p. 265). This 

 opening cannot be said to be into either diacoele or aula but 

 rather, anomolous as it may seem, to mark the boundary be- 

 tween the two, for the cells next it on the caudal side belong 

 to the diencephal, on the cephalic side, to the aula. As Eycles- 

 hymer noted, the paraphysis is separated from the epiphysis for 

 some distance, the precommissure, habense and a stretch of 

 endj'ma and a plexus intervening between their openings 

 (Fig- 6, 52). 



The dorsal sac of fishes is a well known structure, it is large 

 and conspicuous, and the pallium, the membranous roof of the 

 prosencephal, is beneath, and united with it. 



In the amia the pallium (Fig. 93) is a membrane overlying 

 which are great numbers of blood vessels, with branches into 

 the intercerebral plexus (auliplexus) and the intercerebral pia 

 in a way perfectly comparable to the arrangement of vessels 

 from the supraplexus of diemyctylus (p. 265). Partly thrust 

 into this mass of blood vessles and convoluted by them — partly 

 overlying them, is the dorsal sac, which as seen from the meson 

 has a narrow connection with the cavity (Fig. 93, /ar.), but in 

 transection has a wide orifice (Fig. 98). It is a pocket or sac 

 of endyma reflected from the supracommissure with a diver- 

 ticulum extending caudad over the supracommissure and 

 habense (Fig. 99). The stalk of the epiphysis arising as it 

 does caudad of the supracommissure is thus brought into con- 

 tact with the dorsal sac and continues cephalad upon it or 

 partly enclosed in it (Fig. 98) and from this arrangement the 

 term second vesicle of the Epiphysis has been applied {Zirbel- 

 polster of German writers). 



As in amia and lamprey there is no partition dipping ven- 

 trad, between the prosen- and diencephal in which the para- 

 physis may be sought, the essential relations of the above parts 

 must be considered. The pallium with regard to its vessels is 

 comparable to the supraplexus, and its endyma to the mem- 

 brane from which the paraphysis rises in the diemyctylus, (see 



