456 



Attached to this epithelial layer, and lying in the cavity of the dorsal 

 sac between the folds of the choroid plexus, is a cytoplasmic net- 

 work containing numerous nuclei and apparently composed of extrusive 

 connective-tissue cells. The choroid plexuses of the fourth and lateral 

 ventricles are practically identical in histological structure with that 

 of the dorsal sac. 



The paraphysis grows upwards immediately in front of the dorsal 

 sac, and its upper end turns backwards over the roof of the latter. 

 It must be regarded as a compound tubular gland. Its walls become 

 greatly folded, and in the adult we find a central lumen surrounded 

 by numerous crypts and opening into the dorsal sac. Between the 

 crypts numerous blood spaces develop, which sometimes form a regular 

 network of thin-walled sinuses or capillaries. These are supplied with 

 blood by paraphysial branches of the saccular and anterior choroidal 

 arteries, and drain into the sinus longitudinalis beneath the pineal sac. 



The paraphysis is invested by pia mater, which attaches it firmly 

 to the dorsal sac. Its epithelial lining has a very characteristic histo- 

 logical structure, consisting of a single layer of cells without distinct 

 boundaries, and connected together by radiating threads of cytoplasm 

 to form a syncytium. In connection with this epithelium there is a 

 very conspicuous but irregular network of nucleated cytoplasm lying 

 in the paraphysial lumina. The nuclei in this network are very poor 

 in chromatin and undergo amitotic division. Sometimes little rounded 

 knobs, covered with the syncytial epithelium, project from the wall of 

 the paraphysis into its various cavities. 



I have already described the origin of the two pineal organs from 

 the brain-roof, and how, from its first appearance, the one which is 

 destined to give rise to the pineal eye usually lies a little to the left 

 of the other, which will give rise to the pineal sac. I am now able 

 to confirm Schauinsland's subsequent observation that these two 

 vesicles are at first in open communication with one another, but I 

 do not consider that this need prevent us from regarding them as 

 members of an originally symmetrical pair, and fresh evidence in 

 favour of this view is put forward in the present memoir. 



The opening of the pineal sac into the third ventricle, between 

 the superior and posterior commissures, closes up at a comparatively 

 early date, but vestiges of the connection remain in the "infra-pineal 

 recess" and in the "pineal tract" by which the pineal sac of the adult 

 remains connected with the brain-roof. The pineal sac grows upwards 

 in close contact with the posterior wall of the dorsal sac, to which it 

 is firmly attached by the pia mater, and its upper end turns forwards 



