PARAPHYSIS AND PINEAL REGION IN REPTILIA 321 



vessel and is at this stage almost completely surrounded by the 

 sinus which has expanded and been more or less broken up to 

 enclose it. This shows a very intimate relation between the pa- 

 raphysis and these veins, the walls of the paraphysis lying in close 

 contact with the cells in the walls of the veins. This big sinus 

 is still further broken up into a very complicated net-work of 

 smaller vessels over the top of the post velar arch which were so 

 small and irregular that no attempt was made to model them. 

 These vessels at a later stage share in the formation of the choroid 

 plexus which develops by the infolding of the top of the arch. 

 On reaching the epiphysis the vascular net-work surrounds that 

 structure closely and behind it becomes concentrated into a large 

 single median vessel overlying the mid brain. The epiphysis 

 and pineal eye are practically the same as in figure 10, but their 

 increased caudal inclination is probably due to some distortion 

 of the model at this point. The recess in the upper dorsal aspect 

 of the post velar arch is very marked above the superior com- 

 missure and is, I think, somewhat exaggerated by the distortion 

 mentioned above. The posterior commissure is steadily increas- 

 ing in size and the angle between its two parts is becoming more 

 marked. The position of its anterior part and that of the superior 

 commissure with reference to the habenular flexure at the base 

 of the brain and also the position of the hinder end of the mid 

 brain, M., should be noted and compared with the previous figure, 

 fig. 10. There the two commissures and the opening of the epiphy- 

 sis lie directly over this flexure, and the caudal limit of the mid 

 brain is approximately on a plane with the optic commissure, 0. C. 

 In fig. 11 the walls of the mid brain are greatly thickened, the 

 cavity much reduced in size, and the whole of this part of the brain 

 has shifted upward and forward so that its caudal limit is now 

 on a plane much above the optic commissure. The anterior end 

 of the posterior commissure and the superior commissure, to- 

 gether with the stalk of the epiphysis, are carried forward so as to 

 lie over the hypophysis, H. This change in position of the mid 

 brain has tended to compress the roof of the diencephalon, as 

 shown by the appearance of a marked angle in the post velar arch. 

 The choroid plexus of the lateral ventricle, L. C. P., is represented 



