256 HILL. [Vou. IX. 
directed dorso-cephalad, and appears to recede from the 
ectoblast and pass ventrally with the transverse fold that 
separates the prosencephalon from the thalamencephalon. 
Its walls are identical in structure with the adjacent brain- 
roof, and consist of a single layer of columnar cells with 
numerous fine cilia on their ventral ends. In position and 
method of growth this structure seems to correspond to 
Selenka’s paraphysis. In Amia 15 mm. long there is nothing 
in the histological structure to distinguish it from a simple fold 
in the brain-roof. It may be thought of .as an isolated portion 
of the roof of the fore-brain which owes its existence to the 
formation of the folds marked Pl. chr. in Fig. 20, and which 
are themselves the representatives of the choroid plexuses of 
the lateral ventricles. These fold into the cavity of the fore- 
brain, and are continuous on each side of the median plane with 
the previously formed fold which separates fore-brain from 
‘tween-brain. They are also continuous with one another in 
the median line in front. The portion of the fore-brain 
roof included between them, and thus isolated and apparently 
elevated through no activity of its own, is the structure marked 
«Par. in Bios, 18 and’20) Its.absence im Teleostss, corre 
lated with the absence of a choroid plexus of the fore-brain. 
If it is the homologue of the paraphysis, so briefly described by 
Selenka and Eycleshymer, then, as Eycleshymer has suggested, 
it may represent an early condition of the ‘true choroid plexus 
of the lateral ventricles,” 
CONCLUSIONS. 
During the progress of this work I have examined nearly all 
of the literature on the epiphysis in every class of vertebrates, 
but so much still remains to be done on the development of 
these organs and on their relation to the central nervous 
system, that the time does not yet seem ripe for an exhaustive 
criticism. I shall therefore content myself with the examina- 
tion of a few points more immediately connected with my 
work. 
