DORSAL VENTRICULAR RIDGE 489 



is clearly recognizable in the rostral part, back to the level of 

 the caudal border of the foramen interventriculare. 



Between the pallial thickening and the cells of the dorsal 

 ventricular ridge appears in the sections a clear space filled by 

 fibers. In the caudal ]3art of the hemisphere, where the dorsal 

 ventricular groove is already formed, this bundle of fibers is 

 seen to hold the same position as an important bundle in the 

 adult. In the adult many fibers from the crus enter this bundle 

 or, more probably, pass through it on their way to the pallium. 

 Here in the embryo the crus is not yet sufficiently develo])ed 

 to enable one to trace it up through the stiiatum to this le\'el. 

 The strong development at this stage of the bundle here men- 

 tioned indicates that it is comjiosed chiefly of fibers connecting 

 parts of the hemisphere; an association bundle, in other words. 



Relations of dorsal ventricular ridcje to basal structures. In 

 the rostral part of the hemis]ihere the ridge is from the first 

 clearly marked ofi" from the stiiatum below by the middle groove 

 and by a prominent cell-frcn^ zone. In later embryos this cell- 

 free zone ])econies comjiressed into a nanower line as seen in 

 section and scattering cells are found in it. There is. however, 

 always a clear boundary between the ridge and the developing 

 lentiform nucleus. In section the cell-free zone inclines dorsad 

 toward the lateral surface and ]iasses al)ove the lateral olfac- 

 tory area. In the rostral i^art, where the i)yriform lobe crowds 

 far dorsal this limiting zone is bent into a semicircular or V- 

 shajKHl line (figs. 23 to 27). When the writer first studied 

 the adult turtle brain he found it im])ossible to establish a direct 

 comparison between the lateral zona limitans of the selachian, 

 frog and other simpler vertebrates, and either of the cell-free 

 zones seen in the lateral wall of the turtle brain. It is now 

 evident that the clear zone which in the adult brain passes 

 from a point near the middle ventricular gi'oove toward the 

 lateral surface and then bends dorsad to reach the surface in 

 the sulcus rhinalis above the lobus pyriformis, is the zona limitans 

 of the embrj'o and corresponds to the zona limitans lateralis in 

 the selachian and frog. (See Johnston, '15 b, figs. 19 to 22 and 

 compare with '11a, fig. 75). Since the relations of the p^'ri- 



