208 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



secretory membrane. The roof plate of the third ventricle, from 

 the foramina of Monro backwards, becomes modified in a similar 

 manner. It merely forms the ependymal covering of the lower 

 surface of the velum interpositum, also a secretory membrane 

 (Fics. 168 and 169). The anterior part of the roof plate is 

 produced into the cerebral vesicles over the foramina of Monro, 

 and covers the apex of the velum interpositum (Fig. 170). The 

 mesial wall of each cerebral vesicle from the foramen of Monro 

 back to the posterior extremity of the vesicle (Fig. 170), which 

 becomes the tip of the descending horn, is also inflected and 

 becomes a secretory ependyma, covering the velum interpositum 

 and choroid plexus within the lateral ventricles. Into this 

 inflection of the embryonic neural wall spreads the mesoblast, 

 carrying vessels with it. The velum interpositum is thus com- 

 posed of a basis of mesoblast and its intraventricular parts have 

 a covering of the ependyma of the neural wall. 



The ependymal covering of the entire velum is derived from : 



(1) The roof plate of the 3rd ventricle (lower surface) ; 



(2) The roof plate of the foramen of Monro ; 



(3) An inflection of the mesial wall of the cerebral vesicle. 

 The choroid plexus, which fringes the velum in the adult, 



completely fills the cavities of the lateral ventricles, which for the 

 first five months are relatively very large and the containing walls 

 thin. The velum and choroid plexus must play an important part 

 in the development of the cerebral vesicle in the early period of 

 growth. The spread of the vesicles backwards and downwards 

 over the optic thalami (Fig. 173) obscures the original simple 

 relationship of the velum to the brain ; but, when withdrawn 

 from the transverse fissure, the velum is seen to rest on the optic 

 thalami and project within the ventricle from the foramen of 

 Monro to the tip of the descending horn, and that stretch marks 

 the line at which the choroidal inflection took place. 



The fibrous substance of the velum interpositum is con- 

 tinuous with the pial covering of the brain, and also with the 

 edge of the tentorium cerebelli. The veins of Galen are developed 

 in the velum and join the straight sinus in the tentorium. 

 Pressure applied to the veins causes dropsy of the lateral 

 ventricles. 



Development of Commissures. — (1) The Anterior Commissure; 



