BY DR. KLEIN. 57 



skin becomes visibly paler. On resuming the observation, after 

 the lapse of one or two hours, it is found that the pigmental 

 network is as dense, and the processes are as numerous, a* at 

 the beginning of the observation. It is a remarkable fact 

 that the projection of the processes is much accelerated by 

 the application of a drop of croton oil, with the aid of a 

 capillary pipette, to the irritated part. In certain places 

 where the cells are not entirely black, but have a more or less 

 yellowish-Jjrown color, and possess only a few stumpy pro- 

 cesses, these last undergo spontaneous changes of form as 

 regards length and thickness. When the web is irritated, 

 these, like the others, retract their processes altogether. If 

 the circulation is arrested by placing a ligature around the 

 leg, the pigment cells on the same side acquire a brighter 

 color the dull brownish-yellow tint returning with the resto- 

 ration of the circulation. 



In the tail of the tadpole the pigment cells in several respects 

 resemble the ordinary branched cells. The most superficial 

 extend themselves by their processes between the epithelial 

 cells. In the tadpole of the toad, which is distinguished from 

 that of the frog by the breadth and shortness of the tail, they 

 are spindle-shaped, and form by their processes a tolerably 

 regular lattice-work, with nearly rectangular spaces, which is 

 uniformly distributed throughout the tissue ; immediately un- 

 derneath the epithelium, however, there are some cells, the 

 mode of branching of which is dendritic. In fresh prepara- 

 tions, or in preparations with chloride of gold, of the mesen- 

 tery of the frog, a greater or less number of pigment cells are 

 seen in the immediate neighborhood of the large bloodvessels, 

 and especially the arteries, and often form a complete sheath 

 around them. Isolated pigment cells occur also elsewhere in 

 the tissue. With high powers (> T o. 10 immersion) and with 

 dilute acetic acid, it is possible to make oift in fresh prepara- 

 tions of the nictitating membrane and mesentery that the 

 whole cell is not pigmented, the pigment being confined to 

 certain parts of the body and to the axes of some of the pro- 

 cesses. In mammalia, the most varied forms of pigment cells 

 occur in the choroid and sclerotic, from the irregularly formed 

 cells with slight knob-shaped projections containing coarse 

 pigment granules, to cells with regular dendritic branching 

 and fine granules. 



Fat Cells. Fat cells are distinguished from ordinary 



branched connective tissue corpuscles mainly by the fact that 

 tUgy contain drops of fat. When an ordinary branched ct'll 

 undergoes -conversion int<> a fat cell, the change commences 

 by the appearance of small droplets in the protoplasm. I>y 

 the confluence of these with each other a larger drop is formed. 

 As this increases, the protoplasm of the corpuscles is dis- 



