42O EMBRYOLOGY OF THE BRAIN AND SPINAL CORD. 



roof- plate to the extent of obliterating the median ventricular sulcus 

 and completely uniting the two dorsal laminae. The cerebellum 

 is now represented by one continuous transverse ridge arching 

 over the fourth ventricle. It develops slowly in comparison with 

 the cerebrum. The lateral parts develop more rapidly than the 

 median portion after the third month. Thus the hemispheres 

 are differentiated from the vermis. The flocculus is the first 

 lobule to be formed and it reaches a high development in the third 

 month; other lobules of the hemisphere which are more charac- 

 teristic of the human cerebellum, the tonsil, the quadrangular 

 lobe, the superior semilunar and the inferior semilunar lobules, 

 are not fully formed until near birth. The jolium vermis of the 

 worm is developed after birth (Figs. 76 and 80). 



Sulci (Fig. 79). The chief sulci of the worm appear in the 

 third month; with two exceptions, those of the hemisphere de- 

 velop later. The lateral part of the postnodular sulcus is first 

 manifest. In the second month it cuts off a strip of the cerebellar 

 ridge, just above the chorioidal plica, which is the primitive floc- 

 culus. A little later the sulcus extends across the median line 

 and forms the posterior boundary of the nodule. The next sul- 

 cus to develop is the predeclivil sulcus (Fig. 79). It cuts very 

 deeply into the vermis between the culmen and the declive. From 

 the vermis it extends into the hemispheres, where it separates 

 the anterior and the posterior parts of the quadrangular lobe. 

 The prepyramidal and the postpyramidal sulci are formed near 

 the end of the third month (Fig. 79). In the fourth month the 

 hemispheral part of the postdeclivil sulcus is first visible behind 

 the quadrangular lobule (Fig. 76). Soon it becomes continuous 

 through the vermis with that of the opposite hemisphere. It is 

 the beginning of the fifth month before the prepyramidal sulcus 

 is extended into the hemisphere between the tonsil and biventral 

 lobule (Fig. 80). At about the same time the lateral extension of 

 the postpyramidal sulcus establishes the posterior boundary of 

 the biventral lobule. According to O. C. Bradley's study of the 

 rabbit's cerebellum, the postcentral sulcus and the precentral sul- 

 cus aj>pear at about the same time as the postdeclivil. The last 

 important sulcus to appear [is the horizontal sulcus (Figs, 76 



