PARAPHYSIS AND PINEAL REGION IN REPTILIA 329 



in lacerta, it becomes very diffusely expanded over the roof of the 

 mid brain. The choroid plexus of the lateral ventricles, L. C. P., 

 here makes its first appearance and consists of a few folds pushed 

 into the ventricle from the mesial wall of the telencephalon. As 

 was the case in lacerta this plexus antedates any of the others. 

 The velum cannot be seen in the figure as the plexus is in the 

 way. In both the lizard and the turtle the beginning of the 

 plexus and the first trace of the superior commissure appear prac- 

 tically at the same time. This stage corresponds fairly closely 

 to Lacerta muralis of 17.5 mm., fig. 10, in as much as both 

 the lateral choroid plexuses and the superior commissure appear 

 first in both. The shape of the post velar arches is essenti- 

 ally the same, though that of the turtle is more highly and uni- 

 formly arched. 



Fig. 21 shows the brain of an embryo of 16.5 mm. The pa- 

 raphysis forms a long narrow tube curving backward and resting 

 closely against the post velar arch. There are no signs yet of any 

 tubules and it is simply a hollow rounded tube with rather i hin 

 walls. The velum is represented by merely a rounded angle 

 concealed by the plexus of the lateral ventricle. The lateral 

 plexus has become very complicated consisting of a mass of villus- 

 like tufts forming a quadrilateral shaped mass of tissue. The 

 post velar arch is more highly developed than before. Its outlines 

 are very regular, the anterior and posterior walls being almost 

 parallel with each other. It forms a very extensive space at the 

 top of the diencephalon, in the upper part of which small in- 

 growths of the wall can be seen. These mark the beginning 

 of the diencephalic plexus, D. C. P., which at a later stage wdll 

 fill all this part of the diencephalon. The epiphysis like the pa- 

 raphysis is a narrow elongated tube and is closely moulded to the 

 posterior wall of the post velar arch. Its tip is much expanded 

 laterally and the extremities of both these structures are gradually 

 approaching each other. Beyond an increase in size there is 

 nothing remarkable about the superior commissure. A slight recess 

 can be seen in the top of the diencephalon just above it. The 

 posterior commissure is still nearly flat. It forms a broad band 

 of fibers reaching to the stalk of the epiphysis anteriorly and 



THE AMERICAN JOTIRNAt, OF ANATOMY, VOL. 11, NO. 4 



