26 Edivard F. M alone. 



The relationship of these various cell groups of the hypothalamus 

 may be summarized as follows : 



1. The substantia grisea ventriculi tertii is the oldest, least highly 

 developed, and the most diffusely distributed cell group of the hypothal- 

 amus. From it have been differentiated at least two (and probably all) 

 of the four cell groups that lie more or less embedded in it. Certain 

 portions of the substantia grisea appear slightly different from the 

 rest, in that the cells are here crowded closely together. Whether any or 

 all of the cells of the substantia grisea take part in conducting nervous 

 impulses, or whether this group serves merely as material from which 

 more highly differentiated cell groups are formed, is not known. 



2. From the substantia grisea at least two different cell groups have 

 arisen ; these two groujjs of cells have developed along diverging lines. 

 One of these two types constitutes the nuclei tuberis laterales ; it is by 

 far the youngest cell group in the hypothalamus, the first indication of 

 its presence occurring in the lemur. The second cell group wiiich has 

 developed from the substantia grisea is the nucleus tubero^mammillaris ; 

 its cell type differs radically from that of the nuclei tuberis laterales, 

 and it is a much older cell group. 



3. Along the same line as the nucleus tubero-mammillaris, and to a 

 higher degree of histological differentiation, have developed the nucleus 

 paraventricularis and the basal optic ganglion ; the nucleus paraventric- 

 ularis is intermediate between the other two nuclei. From such a 

 short phylogenetic seines as that from man to the cat it is impossible to 

 determine whether all three of these nuclei have developed iiidej)en- 

 dently from the substantia grisea or whether the other two nuclei liave 

 developed from the nucleus tubero-mammillaris and thus only indirectly 

 from the substantia grisea. 



4. A more extensive series of animal forms should enable us to under- 

 stand such relations better; but a study of such material cannot be 

 expected to yield satisfactory results unless the different cell groups 

 be distinguished and unless the question of the relationship as ex- 

 pressed by the cell type be borne in mind. 



5. Valuable results may be expected also from a study of the histo- 

 genesis of these various cell groups as revealed in the later stages of 

 development of the human brain; little, however, is to be expected 

 from such material unless it be fixed and stained witli reference to 

 the demonstration of cell structure in the nervous system, and the 

 methods usually employed are not well adapted to this purpose. 



