1910.] and Adjacent Parts of the Brain in the Tuatara. G31 



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 tlie central canal of the fore- and mid-brain. 

 These are, from in front backwards, (1) the rccessus optici lateraks, 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 prenucleares, 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 sense- 

 organs and the optic lobes, and that each of these pairs of outgrowths 

 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 pre- 

 nucleares 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 arteri,es. The folds of this choroid 

 plexus are covered with an epithelium composed of polygonal cells with 

 well-defined boundaries and conspicuous nuclei. Attached to this epithelial 

 layer, and lying in the cavity of the dorsal sac between the folds of the 

 choroid plexus, is a cytoplasmic network containing numerous nuclei and 

 apparently composed of extrusive connective-tissue cells. The choroid 

 plexuses of the fourth aud 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 histological structure, 

 consisting of a single layer of cells without distinct boundaries, and connected 



