486 .). H. JOHNSTON 



clear iliai the siipci licial layor of (*ells of the olfactory iinvd ex- 

 toiuls II]) oil tlic ouUm" surfMce of the pallium and that it is ac- 

 companied b,y a special bundle of fibers. The crowding of 

 cells in this stage is such that it is impossible to determine whether 

 the pyriform lobe cells have spread or migrated up from the 

 lateral surface in the basal region or have been formed here by 

 the proliferation which at this stage is giving rise to the pal- 

 lium. Farther caudad and in later stages the relations are such 

 as to make the former appear more probable. The ceils of the 

 pyriform lobe probably all come from the layer of indifferent 

 cells seen in earlier stages and formed by proliferation in the 

 lateral and basal region before the pallial proliferation seen 

 in this stage began. Pallium and pyriform lobe are still visible 

 in a section (fig. 14) through the peduncle in which the olfactory 

 formation appears in the dorsal wall. The olfactory formation 

 extends far back on the dorsal surface, as is already well known. 



CHRYSEMYS 



Models were made of the 17 mm. and 28 mm. stages. Most 

 of the chief landmarks of the adult brain are already visible 

 in the 17 mm. embryo (figs. 3, 4, 15, 16, 17). The caudal pole 

 still lags behind the rostral portion in development. The cau- 

 dal pole projects much farther back than in the oldest Chelydra 

 embryo and in the 28 mm. stage the olfactory area (lobus pyri- 

 formis) extends relatively farther back than in the 17 mm. 

 stage. The tuberculum olfactorium is well developed and 

 the ventral groove of the ventricle dips deep into it. The mid- 

 dle ventricular groove (fig. 4 and sections) is very sharp and 

 deep in the middle part of the brain and stops abruptly just 

 caudal to the level of the foramen interventricukre. The dor- 

 sal ventricular groove appears in the 17 mm. embryo (figs. 4, 17) 

 as a slight groove in the lateral wall some distance from the 

 medio-dorsal angle of the ventricle. 



Three sections caudal to the foramen are drawn from the 

 17 mm. embryo and nine sections from various levels of the 

 28 mm. embryo. The internal differentiation has already pro- 

 gressed so far in the 17 mm. embryo that it will be unnecessary 



