ioo Journal of Comparative Neurology 



dorsad, the wide dorsal end becomes folded upon itself forming, 

 especially in the adult, a great number of lobes or diverticles. 

 Its structure easily differentiates it from the surrounding tissues. 

 Its ental or endymal layer, is formed of cubical, thick-walled 

 cells with large, deeply staining nuclei. The ectal or pial layer 

 is thicker than the same in the dorsal sac and takes a deeper 

 stain with the picric acid-fuchsin. The relation of the paraphy- 

 sis to the dorsal sac is clearly shown in both sagittal sections 

 (Fig. 30) and frontal sections (Figs. 27-29). Tracing the struc- 

 tures ventrad by frontal sections we find (Fig. 27) that the nar- 

 row neck of the paraphysis lies just cephalad of the transverse 

 wall of the dorsal sac. A section a short distance ventrad (Fig. 

 28) shows the union of the neck of the paraphysis with the 

 membrane of the dorsal sac and the opening of the former into 

 the diaccele. Continuing ventrad we find the membrane stretch- 

 ing across between the walls of the diaccele with only a slight 

 folding. A frontal section at the portas shows the formation of 

 the various plexuses from this membrane. The same essential 

 relations are shown in a sagittal section (Fig. 30) near the me- 

 son. The dipping down of the dorsal sac to join the mouth of the 

 paraphysis forms the velum and is to be considered the morpho- 

 logical limit of the diencephal. No diaplexus is developed from 

 the velum. The paraphysis opens into the diaccele and we have 

 part of that cavity roofed by the prosencephal. These same 

 relations are shown by Herrick (44) to exist in Cistudo. 



While we find the three outgrowths of the diencephalic 

 roof differentiated by their histological structure as well as form, 

 it is to be remembered that, as the cephalic continuation of the 

 paraphysis passes into the plexuses it becomes histologically iden- 

 tical with the membrane of the dorsal sac. This would seem to 

 point to the fact that the two outgrowths were originally of similar 

 histological structure and that they were secondarily different- 

 iated by some difference of function. The paraphysis is almost 

 completely surrounded by a large blood sinus and has the same 

 relations to the blood supply of this portion of the brain that 

 the endolymphatic sacs bear to the supply in the region of the 



