412 EMBRYOLOGY OF THE BRAIN AND SPINAL CORD. 



are acquiring their adult forms and producing the various fissures 

 and sulci, the walls of the hemisphere vesicle thicken rapidly and 

 give rise to the corpus striatum, the cortex and the medullary 

 substance; but the order of development is not yet understood. 

 The development of the gray and the white" substance contracts 

 the cavity of the hemisphere vesicle to the size of the lateral ventricle; 

 and the cornua of the ventricle are produced by the forward, back- 

 ward, and downward growth of the vesicle in the successive form- 

 ative stages. Little is known of the time at which the cortical 

 neurones are formed; but it would seem that they continue to 

 undergo medullation and to become functional up to a late period 

 of life, and the investigations of Kaes support this inference. 



Fornix. A ridge appears, about the fourth month, on the medial 

 surface of the hemisphere vesicle, along the convexity of the chorioidal 

 fissure. That ridge reaches from the lower end of the hippocampus 

 to the roof of the interventricular foramen. It becomes converted 

 into a bundle of fibers which is continued into the lamina terminalis 

 and then through the lateral wall and floor of the diencephalon 

 to the corpus mammillare. It forms a lateral half of the fornix. 

 The union of the two halves in the lamina terminalis forms the 

 primitive body, or corpus fornicis. The body is extended by the 

 upward and backward growth of the lamina terminalis and by 

 the crossing over of certain fibers from one crus fornicis to the 

 other, which results in the formation of the commissura hippo- 

 campi. 



Another ridge, a slight one, appears about the same time on 

 the opposite lip of the chorioidal fissure. It represents the stria 

 terminalis. This latter ridge and the fornix ridge bound the roof- 

 plate as represented in the hemisphere. This roof-plate extension 

 undergoes no development. It forms the chorioid epithelial 

 lamina of the lateral ventricle, which loosely joins the fornix and 

 the stria terminalis, being folded over the chorioid plexus of that 

 ventricle (Fig. 119). 



Internal Capsule. It is formed largely along the line of fusion 

 between the medial surface of the hemisphere vesicle and the 

 lateral surface of the diencephalon. Motor fibers grow down- 

 ward through the corpus striatum from the cerebral cortex, and 



