434 J. UKAHAJl KEKU. 



developed, and fibres have appeared over most of the cerebral 

 and mid-brain surface. 



The paraphysis has increased in size, forming- a tube about 

 •05 mm. in diameter^ and with walls composed of a single 

 layer of cubical cells. This runs straight forwards and up- 

 wards for a distance of about a quarter of a millimetre, and 

 ends blindly in front. 



Stage 35 (fig. F). — The main new features are the appear- 

 ance of a sharp depression marking off the floor of the mid- 

 brain from that of the hind brain, and the fact that the region 

 of the anterior commissure is undergoing a rotation forwards 

 and upwards, so that it is becoming transferred from the 

 floor of the thalamencephalon to its anterior wall. By this 

 stage the process is still inconspicuous, but later on it becomes 

 very obvious. 



The habcnular commissure is very well developed. The 

 paraphysis opens into the cavity of the thalamencephalon 

 almost vertically under the root of the pineal body. The 

 paraphysis has altered its position, having become rotated 

 backwards so as to lie flat against the lamina terminalis 

 almost vertically. 



Stage 38. — The changes seen in sagittal section are com- 

 paratively trivial. The paraphysis is, however, relatively 

 reduced, and the mid-ljrain is more sharply marked off from 

 the hind brain both in roof and floor. 



Development of the Various Brain Regions. 



It will now be convenient to run over the main points 

 in the development of each of the regions of the brani in 

 turn. ^ 



T h a 1 a m e n c e p h a 1 o n. 



We have already seen that the thalamencephalon begins 

 to Ijc marked off from the regions behind it as early as 

 Stage 25, and it rapidly assumes its permanent features, 

 n^hc roof is for a long tiiiu' ])erfectly coiij iniioiis willi tli;il of 



