THE DKVEJiOPMKNT OF I,K 1'1D08IREN PARADOXA. 431 



growth of the hind braiu on each side is seen, the elongation 

 of the mesothalaniencephalic region and its distinct division 

 into a mesencephalic part behind, and a thalamencephalic 

 portion in front, the latter thin-roofed, and forming a kind of 

 pillow or cnshion (pineal cushion), npon which the pineal 

 body rests. Lastly, the elongation of the cerebral hemispheres 

 has now begun. 



In the brain of Stage 38 (fig. 8 c) this elongation of the 

 cerebral hemispheres, of the mid-brain, and of the lateral 

 angles of the hind brain is seen to have gone on still further. 



Finally, in the adult brain (fig. 8 d) the great elongation of 

 the antero-posterior axis is very obvious, affecting all regions 

 of the brain except the thalamencephalon. It will be noticed 

 that the lateral angles of the hind brain have lagged behind 

 in this lengthening, so that now they do not project forwards 

 at all. A curious shifting of the point of origin of the 

 olfactory nerves is also seen to have taken place in the later 

 stages of development, the point of origin in the adult being 

 terminal and anterior instead of external and some distance 

 back from the front end of the hemisphere. 



Changes in the Sagittal Section of the Brain 

 during Development. 



It is very instructive to compare sagittal sections of the 

 brain taken at various stages of development, and in Text-fig. 

 2 I give figures of a series of such sections. The outlines 

 of the sections are from camera drawings. 



In Stage 25 (fig. A) a slight elevation of the brain fioor is 

 seen ; this marks the hinder boundary of the thalamencepha- 

 lon, and the portion in front of it corresponds with what 

 will later on be the fioor and part of the anterior wall of the 

 thalamencephalon. 



At Stage 28 (fig. B) the posterior boundary of the thala- 

 mencephalon is more strongly marked : the floor of this part 

 of the brain shows a slight thickening. 



Within the next two stages (cf. fig. C) c()nsideral)le 



VOL. 46, PAHT 3. NKW SERIKS. D D 



