DORSAL VENTRICULAR RIDGE 487 



to speak of it separately. In both sts.ges many features of 

 the sections can readily be compared \\'ith the structures seen 

 in corresponding sections of the adult brain. For this pur- 

 pose the reader should consult the description of the cell masses 

 in the brain of Cistudo in this Joui-nal for October, 1915. The 

 necessary description of indi\'idual sections is given in connec- 

 tion with the figures. Here I shall take up the relationships 

 of the lateral margin of the pallium and the dorsal ventricular 

 ridge. 



Corpus striatum und lateral olfactorj/ area. In the younger 

 embryos the loosely arranged cells of the lateral olfactory con- 

 fers cover the outer surface of the caudate and lentiform nuclei 

 and form a continuous layer over the lateral surface of the 

 hemisphere (except dorso-caudally). This is shown in figures 

 7 to 12, and this interpretation is given to the models. In the 

 28 mm. embryo there is a small area ojiposite the foramen in 

 which the striatum and the crus bundle contained in it come 

 to the surfiice (fig. 21) and the olfactoiy centers form the pyri- 

 form lobe above and the diag(jnal band below. This small 

 area is painted blue in the model and appears white in figure 5. 

 Sections show that it is the elbow of the crus, with certain large 

 cells contained in it ('15 b, ]). 405), which comes to the surface 

 here. This is j^resinnably due to the rapid growth of both 

 ascending and descending fibers in the crus. The crowding 

 thus ]:)roduced leads to the shifting of the cells of the olfactory 

 center upward and downward. This process must go much 

 further in later development until the large striatal area is left 

 exposed in the adult. The embryonic history thus seems to 

 supi)ort very clearly the hypothesis ('15 b, p. 428, 429) that 

 the exj)osed striatal area in the turtle had been covered at an 

 earlier stage of evolution by the lateral olfactory area. 



Relations of pyriform lobe and pallial jnargin. In the caudal 

 half of the hemisphere the pja-iforin lobe forms a layer external 

 to the corpus striatum and extends up to a thin edge superficial 

 to the dorsal ventricular ridge and the margin of the pallium. 

 This overlapping of the pallial margin by the pyrifoi'in lobe 

 persists in the atlult. At about the level of the interventricular 



