104 THE MICROSCOPE. 
nucleus to its own cell very difficult to make out. The fact that 
each cell has two nuclei would probably be overlooked, were it not 
that the developing cell shows them so plainly. 
From the foregoing description of the origin and method of 
development of the fat cell of the frog, it seems admissible to formu- 
late the following : 
I. That, in the frog, the fat cell is derived from a special plasma 
cell by a peculiar method of development. 
II. That, as the mode of development of the fat cell of the frog 
differs from that described for certain other animals, the process of 
development of the fat cell is not the same in all animals. 
III. That, the development of the fat cell of the frog from a 
special plasma cell, and the aggregation of these cells in a particular 
locality (organ, corpora adiposa), for a special function, (making, 
storing and supplying fat), shows that the fat cell of this animal is a 
fully differentiated cell, having nothing in origin or function in com- 
mon with connective tissue. 
Methods of Investigation.—For studying the development of 
the fat cells, the fat bodies taken from a frog killed in the early 
spring should be used. Careful disassociation with needles in 
normal salt solution, and subsequent staining with picro-carmine is 
useful. It is best, before preparing a body for sectioning, to 
examine a portion of the adjacent one in the manner above 
mentioned, in order to ascertain if it is in suitable condition. If it 
is found that all the cells are full of fat, another frog should be 
tried. 
Hardening with one of the osmic acid fluids, and subsequent 
staining with carmine, gives excellent results, particularly in demon- 
strating the fat, as it is blackened by the acid. 
In staining with borax-carmine care must be used not to remove 
too much of the color with acid, or the cell body will be rendered 
indistinct. 
I have obtained the best results, and sharpest pictures of the 
cells in all stages of development, by hardening the corpora adiposa 
in Miiller’s fluid and alcohol, staining in bulk in alum carmine fluid,* 
and interstitially imbedding in paraffin. With asledge microtome 
a great many sections may be cut from a single body so treated, each 
one of which will show a great number of cells in different phases 
of development. 
*'‘*Tae Microscopge,’’ March, 1888, page 83. ‘‘An Alcoholic Alum Carmine Stain,”’ 
