406 



EMBRYOLOGY OF THE BRAIN AND SPINAL CORD. 



on. It also contains a narrow prolongation of the roof-plate. 

 This prolongation of the roof-plate undergoes very little develop- 

 ment. At first it is placed in the roof of the hemisphere vesicle; 

 but the upward growth of the hemisphere shifts it to the medial 

 surface and, later, the backward and downward extension of the 

 hemisphere carries the roof-plate through a horse-shoe curve 

 down toward the pole of the temporal lobe. No thickening occurs 

 in this prolongation. Becoming indented longitudinally it forms 

 the floor of the chorioidal -fissure in which is developed, from 





JY2 



, 



Fig. 1 1 8. Median section of embryonic brain of the third month. 

 (McMurrich after His.) 



I. Myelencephalon. II. Metencephalon: i, Pons, 2, Cerebellum. ITJ. Isthmus rhomb- 

 encephali. IV. Mesencephalon: i, Pedunculi, 2, Corpora quadrigemina. V. Diencephalon: 

 i, Pars mammillaris hypothalami, 2, Thalamus, 3, Epithalamus. VI. Telencephalon: i. Pars 

 optica hypothalami, 2, Corpus striatum, 3, Khinencephalon, 4, Neopallium. 



ingrowing mesoblast, the chorioid plexus of the lateral ventricle 

 (Fig. 119). 



Rhinencephalon (Figs. 17, 118 and 120). In the fifth week 

 a hollow diverticulum grows out from the antero-inferior wall of 

 the hemisphere vesicle and, forms a prominent lobe. It preserves 

 a lobular form in the horse, and in some other animals; but in 

 man it soon becomes constricted by the sulcus parolfactorius 

 posterior into an anterior and a posterior part, and loses its ven- 

 tricular cavity. The anterior part develops the olfactory bulb, 



