PARAPHYSIS AND PINEAL REGION IN REPTILIA 319 



ever, still in close contact with it and with the brain roof. The 

 epiphysis itself is of oblong shape and its opening still communi- 

 cates with the cavity of the diencephalon. The posterior com- 

 missure does not occupy all of the roof of the synencephalon and 

 there is still a distinct interval between it and the epiphysis. 

 The greater part of this tract, however, lies in the diencephalon. 

 but its hinder end is beginning to overlap onto the mesencephalon 

 and blend with the outer layers of its wall. In fact, from this 

 stage on, the posterior limits of the commissure become more and 

 more diffuse and are indicated arbitrarily in the following models. 

 Fig. 10, Lacerta muralis of 17.5 mm. The growth of the pa- 

 raphysis is very striking. It is now a large rounded tube ending 

 in three distinct tubules and following the direction of the post 

 velar arch, against which it is closely moulded. The opening of 

 the organ is much smaller than in the previous stage and lies 

 immediately anterior to the velum. It seems probable that all 

 the primary anlagen — one, two or three, as the case may be — are 

 eventually taken up and absorbed into one large tube. It is 

 conceivable that the three terminal tubules here may represent 

 three primary outgrowths. This must naturally be merely a 

 matter of conjecture, and probably individual cases vary too 

 much to make any such comparisons of real value. One does 

 not see the velum becatise it is covered up by the anlage of the lat- 

 eral choroid plexus, L. C. P. If the plexus were removed from the 

 model, the velum would appear about as it does in fig, 9. This 

 is the earliest stage in which I have been able to identify the plex- 

 uses of the lateral ventricles. They develop from the roof of 

 the telencephalon anterior and lateral to the paraphysis and in- 

 vaginate the dorso-mesial wall of the hemisphere, see fig. 25, 

 Chrysemys marginata, where the relations are essentially the 

 same. They show already a tendency to form little villus-like 

 projections and are the first of the plexuses to make their appear- 

 ance. The post velar arch has increased in height as well as 

 antero-posteriorly, forming now a high dome-like roof to the 

 diencephalon, with a deep recess at its postero-superior part. 

 The shape and relative extent of the post velar portion of the 

 diencephalon at this stage should be carefully noted. Until now 



