488 J. H. JOHNSTON 



foniiueii the i)yrif()rni lobe begins to extend higher on the hiteral 

 surface. At the same time thickening of the hiteral border 

 of the pallium moves dorsally, becomes crowded and folded 

 on itself and sinks in, forming a projection into the ventricle 

 (figs. 22 to 25). These changes take place rapidly from be- 

 hind forward so that in thirty or forty sections of 10 microns 

 the place occupied by the thick border of the pallium comes 

 to be taken by the pyriform lobe. From this point forward 

 the pyriform lobe forms a characteristic ridge filled with a dense 

 \ayev of cells, very much as in the adult. 



Origin of dorsal ventricular ridge. In the Chelydra embryo 

 above described the dorsal part of the lateral wall is thickened 

 by cells coming from a proliferating area in the dorso-lateral 

 wall. The groove which bounds this thickening was identified 

 with the middle ventricular groove of the adult. In the older 

 embryos of Chysemys the groove is readily identified but has 

 been very greatly deepened by further thickening of the mass 

 just above it. Likewise the thickening of this mass has pro- 

 duced a groove above it, the dorsal ventricular groove. The 

 thick mass, then, is the dorsal ventricular ridge and in earlier 

 stages it is indistinguishable from the dorsal pallium. In the 

 caudal part of the hemisphere of the older embryos the relations 

 of the dorsal ventricular ridge are not essentially changed from 

 those seen in the Chelydra embryo. Caudal to the level of 

 the stria mednllaris (figs. 18, 19) this ridge bulges into the ven- 

 tricle and is covered outside and below by the thick mass of 

 the nucleus of the lateral olfactory tract. In the caudal pole 

 the ridge merges gradually with the general palhum caudal to 

 the level at which the pyriform lobe ends In the rostral part 

 of the hemisphere (figs. 22, 23) the dorsal ridge has become prac- 

 tically independent of the pallial thickening, very much as it is 

 in the adult. From these embryos it is quite clear that the 

 dorsal ventricular ridge arises as a thickening of the lateral 

 border of the pallium and that its continued growth causes 

 ic to project into the ventricle and produces the middle and dor- 

 sal ventricular grooves. In the 28 mm. embryo the mass of 

 cells which in the adult was called the core-nucleus of the ridge 



