546 



TEXT-BOOK OF EMBRYOLOGY. 



thalamus on the ventricular surface, and between medial hemisphere wall and 

 thalamus externally (Fig. 477). The effect is much the same whether accom 

 plished by apposition and fusion or by interstitial thickening, massive con- 

 nections being formed which consist mainly of fibers connecting hemispheres 

 and thalamus, the foramen of Monro at the same time being changed in form 

 to a slit. From the metathalamic region the fibers of the optic and acoustic 

 pathways grow forward into the hemispheres (see also p. 537), entering more 

 caudally and forming the retro- and sublenticular portions of the internal capsule 

 (comp. p. 537). That part of the thalamic radiation from the anterior portion 

 of the thalamus (fillet pathway) also forms a part of the internal capsule as 

 described on p. 537. Later, the internal capsule is completed by the growth 



Medial wall 



Caudate nucleus 



Internal capsule 



Lentiform nucleus 



Lateral wall 



Chiasma 

 Recessus infundibuli 



Chorioid fissure 

 Mesencephalon 



Pedunculus cerebri 



Cerebellum 



Myelencephalon 



FIG. 478. Lateral view of the brain of a 3 months' (42 mm.) human foetus. The lateral wall of 

 the left cerebral hemisphere has been removed. His, Kollmann. 



from the pallium of descending fibers from the neopallial cortex, through 

 the striatum to the pes. By these various traversing fibers the striatum is 

 divided into the nucleus lenticularis or lentiformis and the nucleus caudatus. 

 The posterior arm of the internal capsule is formed by fibers passing between 

 and thus separating thalamus and lenticularis (Figs. 477 and 478). 



THE ARCHIPALLIUM. 



During the fifth week, following the stage shown in Figs. 471 and 472, 

 the pallial evaginations or hemispheres have become much more pronounced 

 and consequently the foramina of Monro much better defined. A comparison 

 will show that the boundaries of the foramen of Monro are essentially unaltered. 

 Anteriorly it is bounded by the medial wall connecting the two hemispheres, 

 posteriorly by the boundary between pallium and thalamus, ventrally by the 

 corpus striatum and junction of it and thalamus (Figs. 463 and 479). 



