Nuclei Tuberis Laterales and the Ganglion Opticum Basale. 25 



relationships may, however, be established. Of these three cell groups 

 the basal optic ganglion is composed of cells whose type is by far the 

 most constant in the different animal forms, and the volume of this 

 cell group is reduced in the lower forms to a less extent than is true of 

 the two other nuclei; moreover, while the cell tj-pe of the basal optic 

 ganglion approaches closely that of the nucleus paraventricularis. there 

 are no transition forms between its cell type and that of the nucleus 

 tubero-mammillaris or that of the substantia grisea; finally the basal 

 optic ganglion is almost entirely free from the invasion of neighboring 

 cells. All of these considerations make it seem probable that of the 

 three nuclei the basal optic ganglion is the oldest and most highly 

 developed. Closely related to the basal optic ganglion is the nucleus 

 paraventricularis ; it is more reduced in size in the lower forms than 

 is the basal optic ganglion ; between its cell type and that of the nucleus 

 tubero-mammillaris transition forms occur, and between its cells lie 

 many cells of the substantia grisea. While these facts make it probable 

 that the nucleus paraventricularis is possibly not so old a cell group as 

 the basal optic ganglion, they enable one to state positively that the 

 nucleus paraventricularis is related on the one hand to the basal optic 

 ganglion and on the other to the nucleus tubero-mammillaris. The third 

 cell group, the nucleus tubero-mammillaris. is like the nucleus paraven- 

 tricularis much reduced in size in the lower animals ; it differs strik- 

 ingly from both the other cell groups in being not sharply circum- 

 scribed, but on the contran- is very diffuse,"^ extending through a large 

 portion of the h}-pothalamus ; it is invaded accordingly by the cells of 

 the substantia grisea to a much greater extent than is the nucleus para- 

 ventricularis. Finally the cell type of the nucleus tubero-mammillaris 

 shows a transition on the one hand to that of the nucleus paraventric- 

 ularis and on the other to that of the substantia grisea. Therefore the 

 nucleus tubero-mammillaris, while related to the nucleus paraventricu- 

 laris, is related to the substantia grisea in three ways : transition forms 

 of cells occur, the cells of both groups are intimately intermingled, and 

 both cell groups are widely and diffusely distributed. The nucleus 

 tubero-mammillaris is therefore the least highly developed of the three 

 groups (owing to its relation to the substantia grisea), but whether it is 

 also the most recent group cannot be decided here: it might just as 

 well be an old and relatively stationary group, and to decide this point 

 a longer phylogenetic series must be studied. 



