326 JOHN WARREN 



The drawing has been shghtl}^ distorted in order to show this. 

 Once in the lateral ventricle this narrow rounded mass expands 

 into a flattened mass of tissue terminating in a complicated, 

 tufted extremity. Its shape is quite characteristic, being longer 

 from above downward as was the case in all its previous 

 stages. This plexus together with that at the top of the dien- 

 cephalon are the only forms of plexus present in the specimen 

 studied. Fig. 14 is a transverse section along the line A-B, fig. 

 13, to show the tubules of the paraphysis. It is somewhat torn 

 in the region of the superior commissure, the position of which only 

 is indicated. Fig. 15 is a corresponding section along the line 

 C-D to show a section of the epiphysis and the diencephalic 

 plexus. It is also somewhat damaged in the same region as in 

 the previous figure. 



Chrysemys marginata 



The models are enlarged only 80 diameters as the older stages 

 were too large to draw conveniently at a greater magnification. 



Fig. 16 shows the fore and mid brain of Chrysemys marginata 

 of 3.2 mm. The line of demarcation between fore and mid brain 

 is indicated by a groove on the dorsal, and a slight ridge on the 

 inner side of the brain, similar to that in fig. 2. The roof of the 

 fore brain forms a simple arch, there being no sign as yet of the 

 velum. At its caudal end immediately anterior to the mid brain 

 is seen a slight arch, >S, corresponding to a similar arch in fig. 2. 



This marks the roof of the second diencephalic segment. In 

 the lizard this appeared at about the same time as the velum and 

 consequently all three subdivisions of the fore brain were developed 

 together. Here there is no sign of any demarcation between 

 the telencephalic and first diencephalic segment. Consequently 

 the second diencephalic segment seems to antedate the appear- 

 ance of the other two. Fig. 17 is from an embryo of 4.8 mm. 

 The velum, V, is well marked and continued into a ridge passing 

 down behind the optic stalk, thus separating the telencephalic, 

 T., from the first diencephalic segment, /. D. Two well defined 

 ridges also mark the boundaries of the second diencephalic seg- 



