534 



appears as a thickening of the lanuina terminalis) its homology is 

 established. 



Of greater interest is the behaviour of the dorsal portion of the 



paraterminal body (Fig. 15, P). In a section made immediately in 



front of the lamina terminalis (Fig. 15) it is still a plump mass of 



m, , ,1 r, u • nerve cells. But as 



Thalamencephalon Paraphysis 



/\ / 



Forraatio 

 pallialis 



Fig. 15. 



Thalam- Formatio 



Epiphysis encephalon pallialis 



Para- 

 physis 



soon as the situation 

 of the lamina terminalis 

 is reached the dorsal 

 part of the paraterminal 

 body suddenly becomes 

 replaced by, or reduced 

 to, a thin epithelial 

 band (Fig. 16, P); and 

 this, again, immediately 

 becomes folded and 

 invaginated into the 

 lateral ventricle to 

 form a choroid plexus 

 (Fig. 17, FL) of the 



Thalamencephalon Formatio pallialis 



J 



Nucleus 

 Taeniae 



Nucleus 

 Taeniae 



Nucleus 

 Taeniae 



Fig. 16. 



Fig. 17. 



Fig. 15. A diagram of another section (20. 2. 4) of the same series cut imme- 

 diately in front of the lamina temiiualis. (140 A. 20. 2. 4.) X ^a. 20. 



Fig. 16. Here the lower parts of the paraterminal bodies have fused to form thej 

 commissure-bed or matrix for the cerebral commissures. The dorsal portion of the para- 

 terminal body, marked P in Fig. 15, has now become attenuated to form an epithelial 

 membrane P. (140 A. 20. 3. 6) 70 jj, behind the section represented in Fig. 15. 



Fig. 17. Diagram of a section 40 [x behind that shown in Fig. 16. (140 A. 

 21. 1. 2.) The paraterminal lamella iP in Fig. 16) has now given place to the lamina 

 chorioidea, which is invaginated into the lateral ventricle to form the choroid plexus {PL.). 



F.3I. Foramen of Monko. vent. Ill third ventricle. 



