DORSAL VENTRICULAR RIDGE 483 



of cells at successi^'e periods of development. Thus the pro- 

 liferation of cells in the dorsal wall gives rise to pallium, that in 

 the basal wall gives rise to various olfactory centers, coi-pus 

 striatum, etc. Also, certain centers owe their origin to the 

 prohferation of cells in an early stage, other centers to prolifera- 

 tion in later stages. The former come to he superficial to the 

 latter and the two are separated by cell-free zones. A further 

 factor is the shifting or spreading of cell masses, whether due 

 to cell-migration or to mechanical forces. These several fac- 

 tors must be taken into account in any attemj^t to use the em- 

 bryological method in tlie study of tlio significance of cell-group- 

 ing in the brain. 



(;ni;i.vi)i;A si;i{Im:.\ tixa 



In the oldest Chclydra embryo studied, it is possible to recog- 

 nize several of the important cell masses and fiber bundles of 

 the telencejohalon and in part the boundary lines between jial- 

 lial and basal areas. Examination of the model (figs. 1, 2) and 

 of sections (figs. 6 to 14) shows that the telencejihalon and dien- 

 cephalon are narrow from ^itle to side and higii dorso-ventrally. 

 The hemisphere is a simple sac which ])rojects much farther 

 rostrad than caudad. The intei-ventricular foramen is still 

 very large. Behind it, the tliin wall which rei)resents the choroid 

 plexus extends nearly half way to the caudal ])ole. This stage 

 illustrates very clearly, both in its general form and its inter- 

 nal structure, how much the rostral \)o\e of the hemis}ihere 

 precedes the caudal pole in de\'elopment. The outer surface 

 of the brain shows few of the landmarks wliich are seen in later 

 embrj^os and adults. The stem-hemisphere sulcus is of course 

 the most prominent. The olfactoiy peduncle is not yet formed, 

 but the broad depression on the lateral surface may represent 

 the beginning of constriction. The fissura prima and diagonal 

 band are recognizable at the medio-basal angle in front of the 

 preoptic recess. Thei-e is a shallow furrow over the rostral 

 part of the foramen which may be the beginning of the sulcus 

 fimbrio-dentatus. 



