423 



surface of the cerebellum from behind, for in this stage of development 

 the greater part of the cerebellum lies intraventricular. The first in- 

 dication of this state is to be found in a model of His ^), representing 

 the cerebrum of an embryo with a rump-vertex-length of 53 mm. In 

 this model the cerebellum shows on each side of the median plane a 

 prominence into the fourth ventricle. Now, in the stage of develop- 

 ment, shown in the embryo of 55 mm these prominences have con- 

 siderably increased. As a consequence of this increment in size they 

 meet in the median plane forming a rafe (Fig. 1 B). 1 should be in- 

 clined to believe, led by the histiological structure of the top of the 

 rafe (Fig. 5), that the two prominences unite in this rafe, thus pro- 

 ducing an increase in 

 thickness of the median 

 zone of the cerebellum. 

 The primitive part of 

 this zone may be re- 

 cognized by the fact 

 that it comprises the 

 dorsal commissure of the 

 cerebellum (Fig. 1 C.d.). 

 By this large intra- 

 ventricular prominence 

 the form of the lamina 

 cerebelH has changed. 

 If we disregard for a 

 moment the tela cho- 

 rioidea, the cerebellum 

 as seen from the side, 

 is triangular. The 

 superior surface, which 

 is the uncovered one, 

 goes over into the 

 postero-inferior surface. 

 At the place of passage 

 we find the insertion 



of the tela chorioidea, so that the whole postero-inferior surface is 

 covered by the superior sheet of the tela chorioidea. This state was 

 first described by His and afterwards by Bolk. The angle at which 



Fig. 2. Frontal section at a distance of 1.0 mm 

 from the posterior border of the cerebellum. 15/1. Out- 

 line of a photograph. Br.c. brachium conjunctivum. 

 C. cerebellum. Isth. isthmus. 3Ies. mesencephalon. P.ch. 

 fissura chorioidea. P.ene.d. plica encephali dorsalis. 

 T. taenia. T.ch.q. tela chorioidea of the fourth ven- 

 tricle. V.c. valvula cerebelli. V.mes. mesencephalic ven- 

 tricle. V.rh. fourth ventricle. 



1) W. His, Die Entwickelung des menschlichen Gehirns, Leipzig 

 1904, p. 92, Fig. 62. 



