455 



of the paraphysial opening their roots of attachment may extend a 

 little in front of and behind the latter. There is no unpaired plexus 

 medianus as described by Gisi. 



The paraphysis, as I have previously shown, is part of the same 

 system of folds of the epithelial lamina supraneuroporica which gives 

 rise to the plexus hemisphserium, but growing outwards instead of 

 inwards. It originally opens into the prosencephalon immediately in 

 front of the commissura aberrans. In an advanced stage of devel- 

 opment, however, a longitudinal supra-commissural canal is formed 

 above the commissura aberrans, and this leads to the formation of a 

 new opening for the paraphysis in the adult, directly into the dorsal 

 sac at some distance above the commissure. The original opening 

 of the paraphysis, in front of the commissura aberrans, is blocked up 

 by the growth of the anterior choroidal veins and arteries. 



There is no true commissura mollis as described by Gisi, though 

 the lateral walls of the third ventricle come into contact with one 

 another over a considerable area. 



Both in advanced embryos and in the adult animal three pairs of 

 lateral diverticula, in addition to the cerebral hemispheres, the optic 

 lobes, and the pineal outgrowths, open into the central canal of the 

 fore- and mid-brain. These are, from in front backwards, 1) the 

 recessus optici laterales, which appear to be remnants of the cavities 

 of the stalks of the optic vesicles; 2) a pair for which I propose the 

 name recessus thalami praenucleares, because they lie in the substance 

 of the optic thalami in front of the nuclei rotundi; 3) the recessus 

 geniculi (of Gisi), which lie on either side of the entrance to the iter, 

 beneath the posterior commissure. It is suggested that these three 

 pairs of diverticula may be serially homologous with one another and 

 with the cerebral hemispheres, the outgrowths which form the pineal 

 «ense-organs and the optic lobes, and that each of these pairs of out- 

 growths indicates an original neuromere. In accordance with this 

 view the cerebral hemispheres would belong to the first neuromere of 

 the fore-brain, the optic vesicles of the lateral eyes to the second, the 

 recessus thalami praenucleares to the third, and the pineal outgrowths 

 to the fourth, while the recessus geniculi would belong to the first, 

 and the optic lobes to the second neuromere of the mid-brain. 



The middle portion of the pineal complex is formed by the thin- 

 walled dorsal sac, the roof of which gives rise to a well- developed 

 choroid plexus supplied by branches of the saccular arteries. The 

 folds of this choroid plexus are covered with an epithelium composed 

 of polygonal cells with well-defined boundaries and conspicuous nuclei. 



