DIENCEPHALON. 415 



cinerea and the small dentate fascia of the adult are the rudimen- 

 tary remains. 



Septum Pellucidum. That part of the lamina terminalis 

 which is included between the corpus callosum and the body of 

 the fomix persists as the septum pellucidum. It develops a lymph 

 space in the median plane, called the fifth ventricle (cavum septi 

 pellucidi) and becomes so thin as to be translucent. 



The above description, giving the origin of the septum pellucidum, corpus 

 callosum and anterior commissure from lamina terminalis, is the one com- 

 monly accepted; but it is not their origin in the rabbit (Marshall) or in the 

 rat (Zuckerkandl), and there is still room for doubt concerning their origin in 

 the human brain. It is possible that all or a part of them are developed in a 

 crescentic area of fusion between the cerebral hemispheres, as in lower animals. 



The pars optica hypothalami (Fig. 118) is developed in the 

 ventral zones of the telencephalon. The tuber cinereum and the 

 infundibulum are true derivatives of these telencephalic zones, 

 but the optic chiasma is not. It is produced by the ingrowth of 

 fibers from the retinae and the medial geniculate bodies. 



DIENCEPHALON. 



This is the posterior division of the anterior primary vesicle. 

 It forms the inter-brain and contains the greater part of the 

 third ventricle (Figs. 17, 118, V, and 119). Its lateral walls pre- 

 sent on each side a distinct ventricular sulcus, the sulcus hypo- 

 thalamicus (Monroi) (Fig. 119, B and C) which separates the ven- 

 tral zone from the dorsal zone (?). 



TABLE VII. 

 DERIVATIVES OF THE DIENCEPHALON. 



Derivatives. 



~ , , . \ Roof epithelium of third ventricle. 



Roof-plate 



I Corpus pmeale (epiphysis). 



i Thalamus 

 Dorsal Lamina { . , 



I Corpora gemculata. 



I Pars mammillaris hypothalami 

 Ventral Zone Corpus mammillare 



( A small part of tuber cinereum. 



I Chorioid tela and plexus of third ven- 

 Surroundmg Mesoblast - . , 

 / tncle. 



