484 .1. n. JOHNSTON 



Tlio \'cntricle shows a xcvy sharp ventral groove leading 

 forward from the foramen and much deepened in the region 

 of the future tuberculum olfactorium. The middle ventricular 

 groove is clearly marked in the region rostral to the foramen, 

 while the dorsal groove has not yet appeared. 



In spite of the simple form of this brain, the internal structure 

 shows considerable advance in differentiation. 



The sections illustrated in figures 6 to 14 can be understood 

 best by beginning with that which passes through the rostral 

 border of the foramen interventriculare (fig. 9). In the medial 

 wall above the foramen are recognized the primordium hippo- 

 campi and the fimbria, one of the early fiber tracts to be formed 

 in the embryo. The dorsal wall is occupied by pallium, the 

 medial portion by hippocampal, the lateral portion by general 

 pallium. In the lateral wall the pallium seems to be limited 

 by a ventricular groove and by a cell-free space. This bound- 

 ary line strongly reminds one of the lateral zona limitans of 

 the selachian and frcg brains (Johnston, '11 a). The later 

 embryos show this groove to be the middle ventricular groove 

 of the adult brain and the clear area becomes the cell-free zone 

 separating the dorsal ventricular ridge and the nucleus lenti- 

 formis in the adult. In the lower half of the lateral wall the 

 following structures are seen in the figure: an active prolifera- 

 tion which gives rise to the nucleus lentiformis, the nucleus 

 caudatus, the lateral and medial forebrain bundles, the anterior 

 commissure bundle, the fiber layer of the diagonal band, and 

 the lateral olfactory area. Although I have not sufficient stages 

 to enable me to follow the history of these structures, there is 

 evidence that the superficial cell layer which represents the 

 lateral olfactory nucleus has been derived from the layers of 

 indifferent cells in the early embryo and that the mass of the 

 caudate nucleus represents a later proliferation. The embryo 

 shows clearly that the lentiform nucleus is beginning to form 

 when the caudate is already completely formed or nearly so. 

 As the other sections are studied it will become evident that the 

 len:iform nucleus is formed by a proliferation distinctly dorsal 

 to the caudate and that the changes in later development bring 



