330 JOHN WARREN 



mingling with the wall of the mid brain behind. Its posterior 

 limit is here arbitrarily shown.The general shape of the cavity 

 of the diencephalon has undergone a marked alteration, although 

 the antero-posterior distance between epiphysis and paraphysis 

 is essentially the same as in the previous stage. The lower or 

 infundibular part of the diencephalic cavity has developed in a 

 ventral direction forming a deep pocket, while the upper part of 

 the cavity, bounded by the post velar arch, has developed in a 

 dorsal direction forming a striking dome-like space at the top of 

 this part of the brain. The hypophysis is not shown in this 

 model. 



Fig. 22 shows the brain of a turtle of 26 mm. In this model 

 the upper half of the lateral choroid plexus, L. C. P., has been dis- 

 sected away in order to give a clearer view of the paraphysis. 

 The organ consists of a large central canal which gives off a com- 

 plicated mass of tubules and is much larger than in the previous 

 stage (see fig. 24). It curves backward over the post velar arch 

 and its apex almost touches the tip of the epiphysis. Lying c^ver 

 the top of both organs is the large superior longitudinal sinus. 

 From this vessel small branches, Ve., pass down on all sides of 

 the paraphysis and intermingle intimately with its tubules. In 

 addition to these veins draining the organ from above another 

 set passes downward from the lower part of the organ and dis- 

 appears in the deep groove between diencephalon and telenceph- 

 alon on both sides. The association between the walls of the 

 tubules and of these veins is most intimate and we have here 

 really a sort of sinusoidal circulation. The relations are better 

 marked than in the lacerta where the development of tubules from 

 the paraphysis is not so complex and approaches more nearly 

 the conditions seen in amphibia. Fig. 27 shows the structure 

 of the paraphysis and the relations of the vessels in a specimen 

 32 mm. in length. The drawing is slightly diagrammatic. The 

 plexus of the lateral ventricles has increased greatly in size and 

 complexity and its long axis is about horizontal. At the hinder 

 border of the mass is seen a tuft of plexus, extending backward 

 into the diencephalon with a curious prolongation stickin up 

 from it (see also fig. 26, T. C. P.). This tuft springs from the ori- 



