500 J. H. JOHNSTON 



Figs. 15, 16, 17 Three transverse sections of the right hemisphere of the 

 17 mm. Chrysemys. Figure 17 falls in the caudal part of the foramen and figure 

 15 represents next to the last section in which the connection of the hemisphere 

 with the thalamus is seen. Figure 16 is between these two. 



In this embryo the mass formed by the pallial proliferation of the earlier stage 

 has become divided into a general pallium and the dorsal ventricular ridge. The 

 ridge projects into the ventricle forming a deep middle ventricular sulcus and a 

 shallow dorsal sulcus. Note that the dorsal sulcus is quite independent of the 

 dorso-medidl angle of the ventricle. The dorsal ventricular ridge therefore is 

 clearly a thickening of the lateral wall above the lateral zona limitans. This 

 zona limitans is the light streak in the drawings running diagonally between 

 d.v.r. above and n.l. and l.pyr. below. The nucleus of the diagonal band is very 

 large and definite in figure 17, while in figure 15 and 16 its substance is fused with 

 the nucleus of the lateral olfactory tract. The lentiform jiroliferation is still 

 in progress, especially from the dorsal lip of the middle sulcus. The peculiar 

 arrangement of ependyma cells seen at this point in the adult already begins to 

 be apparent here. 



Figs. 18 to 27 Transverse sections of the right hemisphere of the 28 mm. 

 embryo of Chrysemys. 



Fig. 18 Section at about the point where the fimbria curves down behind the 

 choroid area. Caudal to this the nucleus of the lateral olfactory tract soon 

 disappears and the dorsal ridge continues on into the caudal pole as a simple 

 thickening of the lateral walls. At g.p.? is a partly segregated mass of cells 

 which probably represents general pallium extending from the caudal pole for- 

 ward to this point in the basal wall as it does in the adult. 



