430 



EMBRYOLOGY. 



it appears as a thick transverse ridye (figs. 242, 243 M), which over- 

 hangs the attenuated roof of the medulla. In the third month the 



middle portion of the ridge 

 acquires four deep trans- 

 verse folds by the sinking 

 in of the pia mater (fig. 

 242), and in this way 

 is distinguished as the 

 vermiform process from 

 the lateral parts, which 

 still appear smooth (kJi). 

 From this time forward 

 the lateral parts outstrip 

 the middle part in growth, 

 bulge out at the sides as 

 two hemispheres, and, ac- 

 quiring transverse folds, 

 in the fourth month be- 

 come the voluminous hemispheres of the cerebellum. 



Only a little nerve-substance is developed where the roof of the 

 fourth brain-vesicle, which has become thickened to constitute the 

 vermiform process and hemispheres, is continuous with the roof of 

 the third and fifth vesicles (fig. 241). Consequently there arise here 

 thin medullary lamellae, which serve as a transition on the one 

 hand to the posterior choroid plexus, and on the other to the lamina 

 quadrigemina (vJi) the 'posterior and the anterior velum medullare. 



Fig. 243. Brain of an embryo Calf 5 cm. long, seen 

 from the side. The lateral wall of the hemisphere 

 is removed. After MIHALKOVICS. Magnified 3 

 diameters. 



csi, Corpus striatum ; ML, foramen of MONHO ; agf, 

 fold of the choroid plexus (plexus choroideus 

 lateralis) ; amf, fold of the cornu Ammonis ; Teh, 

 cerebellum ; Dp, roof -plate of the fourth ventricle ; 

 bb, pontal flexure ; mo, medulla oblongata ; mh, 

 mid-brain (cephalic flexure). 



(3) Metamorphosis of the Third or Mid-brain Vesicle. 

 (Figs. 235, 243, 242, 241.) 



The mid-brain vesicle is the most conservative portion of the embry- 

 onic neural tube, the part which is changed least of all ; in Man a 

 small portion only of the- brain is derived from it. Its walls become 

 rather uniformly thickened on all sides of the cavity, which is narrow 

 and becomes the aqueduct of SYLVIUS. The base and lateral walls 

 together supply the crura cerebri and substantia perforata posterior. 

 The roof-plate (fig. 242 vh] becomes the corpora quadrigemina, 

 owing to the appearance, in the third month, of a median furrow, 

 and, in the fifth month, of a transverse one crossing it at right 

 angles. 



