MORPHOLOGY OF THE FOREBRAIN 475 



('04, figs. 34 and 35). The telencephalic part only of this ridge 

 is present, being seen in the model, but not in this figure, on each 

 side of the median plane along the dorsal part of the margo re- 

 uniens (fig. 82, 1, 1, 1.). At this stage it is not involved in the 

 evagination of the hemisphere. A side view of this model show- 

 ing this feature is published by His ('04, p. 58, fig. 36). In older 

 human embryos, as in most gnathostom.es, the telencephalic part 

 of this ridge is fully evaginated into the cerebral hemisphere. 

 It is incorporated into the primordium hippocampi, to whose 

 morphological interpretation we shall return. 



The diencephalic part of the ciorso-median ridge belongs in the 

 epithalamus. Its fate in gnathostomes is obscure. It appears 

 to be represented in the subhabenular tissue (cf. the Ziegler re- 

 productions of the His models, nos. 1, 4, and 7). 



In the four weeks human embryo (fig. 82) the unevaginated 

 part designated by His C.s. evidently includes both the corpus 

 striatum and the preoptic nucleus, as well as a part of the tissue 

 which when evaginated will form the precommissural body; 

 that is, it includes the ventro-medial parts of the definitive hemis- 

 sphere. The evaginated part includes the remainder of the rhin- 

 encepialon and the pallium. In fig. 82 the dotted line 1-2-2-1 

 marks the limit of the hemisphere evagination and the line 3 the 

 site of the future sulcus medius, which appears in the embryo of 

 about 4^ weeks, after the narrowing of the interventricular fora- 

 men has begun (see His, '04, fig. 38 and the Ziegler model of em- 

 bryo Ko). The sulcus medius in the 4i weeks embryo extends 

 backward to the sulcus limitans and separates the dorsal and ven- 

 tral parts of the thalamus in the amphibian fashion. 



The line 2-2 (fig. 82) marks the beginning of the di-telencephalic 

 fissure, which remains membranous throughout life and connects 

 the epithalamus and pars dorsalis thalami with the pallium of the 

 hemisphere. 



I have examined in this connection a large number of series of 

 mammalian embryos of different species and adult brains of the 

 opossum, Didelphys, and of the rat and conclude that the morpho- 

 logical generalizations reached in the preceding discussion find 

 ready application in these brains without any fundamental change. 



