Chap. XIX.] DURING UTERINE LIFE. 339 



tliin and bag-like at this stage, so that each encloses a 

 very large 'lateral ventricle,' within which a rudimentary 

 Corpus Striatum is to be seen, in the form of a thick- 

 ening of its under and outer wall. Thus it is that these 

 bodies come to occupy their well-known position anterior 

 to and a little outside the Thalami. 



During this same period the ' middle swelling ' or 

 Mesencephalon has not grown at all proportionately, so 

 that it now has a much smaller relative size (fig. 124, c q). 

 It is, however, marked 

 by the appearance of a 

 slight longitudinal fur- 

 row, and its hinder bor- 

 der touches the Cere- 

 bell nm (c'). Its upper 

 walls are comparatively 

 thin, forming the roof of 

 a proportionately large 



cavity, situated between Fig. 12G. -Brain of Human Fcetns, of Fourth 



' J „ , Month, magnified, about two diameters. (Owen.) 



the third and lOUrtn Ven- side view, wltli cerebrum (P) tilted upwards and 



tricleS, though the cavity forwards so as to uncover the corpora qxiadrige- 



' *-• ^ *^ mma (o o), and the bilobed Cerebellum (c c). 



subsequently diminishes 



so as to form a. mere passage between these ventricles. 



The relatively large Medulla still preserves its primitive 

 bend. Its upper half is bridged over by the Cerebellum, 

 while at the back of its lower half is the widely open 

 * fourth ventricle,' the lower part of which is continuous 

 with the central canal of the remaining portion of the 

 primitive tube now developing into the Spinal Cord. 



By the end of the 4th month the principal addi- 

 tional changes which have been noted are these. The 

 Cerebral Hemispheres become still larger and tend more 

 and more to eclipse other parts. They already stretch 

 back over the future Corpora Quadrigemina (fig. 126). 



