I'HK DEVELOPMENT OF LEPIDOSIRKN PARADOXA. 437 



and just posterior to the paraphysis it becomes invagiiiatecl 

 into the third ventricle^ forming a definite complicated velum, 

 continuous on each side Avith the choroid ingrowth of the 

 hemisphere. The velum develops late, but is present in 

 Stage 39. 



The Hemispheres. 



The high development of the cerebral hemispheres, and in 

 particular of the pallium, forms one of the most interesting- 

 features of the brain of Dipnoans. At their first appearance 

 the hemispheres form paired separate bulgings of the lateral 

 walls of the thalamencephalon. There is no forward growth 

 of the anterior median portion of the wall. The hemispheres 

 soon begin to grow up dorsalwards on either side of the 

 thalamencephalon. Later on there sets in marked forward 

 growth, and the immense antero-posterior length of the 

 hemispheres becomes in the adult one of their most con- 

 spicuous features. 



As the hemispheres grow forwards their outer and inner 

 walls become equally thickened, except in their posterior 

 portions. Here, where the inner wall of the hemisphere 

 faces the thalamencephalon, it remains relatively thin. About 

 Stage 35 a small rounded portion of this thin part of the wall 

 bulges into the lateral ventricle. This rounded projection 

 contains a vascular loop : it is the rudiment of the lateral 

 plexus of the hemisphere. The rudiments of the two lateral 

 plexuses are thus seen to be perfectly separate bilaterally 

 symmetrical structures. The plexus rudiment at once begins 

 to grow forwards into the cavity of the lateral ventricle. By 

 Stage 38 it has grown about half the length of the ventricle, 

 and has become swollen out into an irregular crumpled lamina. 

 Finally, in Stage 39 (cf. Text-fig. 2, H), the plexus has 

 become greatly complicated, and fills almost the Avhole of 

 the ventricular cavity. 



