24 Edward F. Malone. 



invertebrates ; it is the only tj'pe in the hypothalamus which approaches 

 that of the neuroglia cells, and transition forms occur concerning which 

 one is in doubt as to whether they are neuroglia cells or cells of the sub- 

 stantia grisea ; the cells are most abundant near the ventricle from 

 whose border all cells of the hypothalamus have arisen, while laterally 

 they are replaced by more highly differentiated types of cells. It is by 

 no means certain that all or even any of the cells of the substantia 

 grisea are functional. That the cells of the nuclei tuberis laterales have 

 arisen from those of the substantia grisea is certain ; if we look in the 

 lemur at the regions where in higher forms the nuclei tuberis laterales 

 occur, we see that the type of cell is somewhat different from the sur- 

 rounding cells of the substantia grisea ; this difference increases in 

 maeaeus, and still more in man, while in the cat there is no indication of 

 any new group. It is thus evident that the most recent cell groups of 

 the hypothalamus (nuclei tuberis laterales) have arisen directly from 

 the oldest cell group (substantia grisea), and that new cell groups are 

 not necessarily formed by the further histological differentiation of 

 portions of cell groups which are already highly developed, thus result- 

 ing in a very highly differentiated cell type, but that on the contrary 

 new cell groups may arise from cell masses which have remained primi- 

 tive. In other words the fact tliat a cell group is of recent origin does 

 not give any information as to the extent of its histological differentia- 

 tion, since this differentiation depends also upon the development of the 

 cell group from which the new group arises. 



In considering the second class of cell groups in the hypothalamus, 

 all of which are composed of large cells, the nucleus ansae peduncularis 

 requires only a brief consideration. The cell t}-pe of this cell group 

 differs considerably from that of other groups, and the nucleus is prob- 

 ably of much more recent phylogenetic origin, since I have not as yet 

 been able to find it in the cat ; in order to consider the relations of this 

 cell group a study of the neighboring regions of the telencephalon will be 

 necessary, and I shall content myself with having shown that it is clearly 

 distinguishable from the basal optic ganglion, and with having given 

 the relative location of both groups of cells. 



Concerning the other three cell groups in this class (basal optic gang- 

 lion, nucleus paraventricularis hypothalami, and nucleus tubero-mam- 

 millaris), the phylogenetic series from the cat to man is far too short 

 to give such satisfactory results as in the ease of the nuclei tuberis 

 laterales, since the former three groups are of much older origin : certain 



