THE DPJVELOPMENT OF LBPIDOSIKEN PAEADOXA. 435 



the mid-brain. The first indication of where the boundary 

 between the two regions will come is given by the appear- 

 ance of the pineal body. Appearing first about Stage 

 31 as a simjDle flattened evagination of the brain roof, 

 and never showing at any time any symptoms of division 

 into a true pineal body and a parietal organ, the pineal 

 body increases somewhat in length, and eventually becomes 

 somewhat carrot-shaped, being attached by its hinder 

 narrower end. Its walls remain simple, and the communi- 

 cation of its lumen with that of the In'ain persists till 

 Stage 38. In the brain of Stage 39 the lumen has become 

 obliterated posteriorly. Anteriorly it still persists, but the 

 cells bounding it show signs of breaking doAvii, and the 

 cavity is to a great extent filled with granular material 

 apparently derived from them. At no time does the pineal 

 body of Lepidosiren show any eye-like characters, and it 

 always lies well down below the skull roof. 



The roof of the thalamencephalon in front of the pineal 

 body, while remaining thin in the median line, thickens out 

 greatly on each side to form the habenular ganglion. These 

 thickenings become conspicuous about Stage 31, and the 

 commissure connecting them — the superior commissure — be- 

 comes conspicuous about the same time. 



The pineal body lying on the roof of the thalamencephalon 

 presses down the thin roof between the habenular ganglia, 

 very ranch as a head does the pillow on which it rests. 

 Consequently the name pineal cushion or pillow (Zirbel- 

 polster) is a thoroughly suitable one for this region of the 

 brain-roof. From the originally anterior wall of the thala- 

 mencephalon is developed the paraphysis, the chief characters 

 of which have been sufficiently brought out in the study of 

 sagittal sections. Appearing first about Stage 31 as a small 

 evagination of the anterior wall of the thalamencephalon at 

 about its middle, it becomes a straight tubular structure, 

 running upwards and forwards between Ihe hemispheres. 

 The paraphysis soon becomes relatively reduced in size. 

 By Stage 39 it forms ;iii ii-reguhirly twisted tu])e, souiewhat 



