550 TEXT-BOOK OF EMBRYOLOGY. 



The longitudinal growth of the hemispheres naturally affects the form of a 

 number of its structures. As already mentioned, this growth consists in an 

 elongation around a fixed point, which may be regarded as located on its ven- 

 tral border, the result of this being a curving down in front and behind this 

 point. This is especially marked in the caudal half which thereby becomes 

 curled first ventrally and then forward, thus forming a spiral. This growth in 

 length is interstitial, i. e., due to expansion of the intermediate parts, and pari 

 passu with it there is an elongation not only of the corpus striatum and 

 structures in the mesial hemisphere wall (hippocampal formation, corpus callo- 

 sum, chorioid plexus of lateral ventricle), but also of adjacent thalamic struc- 

 tures (stria terminalis or semicircularis) , as described later. 



FIG. 474. View of the inside of the lateral wall of anterior part of fore-brain. Human embryo 



of about 4! weeks. His. 



C, Corpus striatum; H, pallium; h. R, posterior olfactory lobe; L, lamina terminalis: O, re- 

 cessus (prae-?) opticus; R. i., recessus infundibuli; 5. M.. sulcus hypothalamicus; St, hypo- 

 thalamus; T, thalamus; v. R., anterior olfactory lobe. 



The early divisions of the corpus striatum have been mentioned, and also 

 the relations of its parts with the rhinencephalon. The anterior end of the 

 corpus striatum at this period and later shows a longitudinal division into 

 three portions, a lateral, a middle and a medial, due to the original division 

 into three limbs described above (p. 545). (Figs. 474, 475, and 476.) With 

 the elongation backward of the hemisphere the corpus striatum also becomes 

 elongated, being drawn out and curled around the peduncle or stalk 'of the 

 hemisphere and forming a thickening along the elongated wall. This caudal 

 prolongation of the striatum is its cauda (tail] and extends to the tip of the in- 

 ferior horn (Figs. 475 and 476). The medial portion of the corpus striatum 

 forms a triangular projection (Figs. 464 and 466) the edge of which is directed 

 toward the foramen of Monro. 



