68 MANUAL OF HISTOLOGY. 



struction of tissues. The rarity of crystals as cell-contents is thus 

 explained by this law, as it may be called, to which, with all the varie- 



Rg. 50. Stellate cells containing black Fig. 51. a b. Crystals of margarin; c, 



pigment. the same contained within fat cells; 



of, a cell from adipose tissue destitute 

 of crystals. 



ties of cells in the different groups of the animal kingdom, there are but 

 few exceptions. 



47. 



Among the further characteristics of the animal cell there now only 

 remains for our consideration the envelope and nucleus. 



4. The envelope. It has been already remarked that the protoplasm 

 at the surface of the cell rarely remains so soft as in the interior. In 

 general terms it may be stated that a hardening of the non-granular or 

 free periphery of the cell usually takes place by contact with surrounding 

 media (enveloping or cortical layer of protoplasm). This hardening is 

 certainly, in numbers of cases, exceedingly slight, so that it is only to be 

 recognised by the sharper outline of the cell: it can also be easily overcome, 

 and softening again brought about by the very brief action of external 

 agencies. In other cases, however, it is greater ; the transparent, tough 

 layer increases in thickness, and may be brought into view as distinctly 

 separable from the richly granular protoplasm of the interior, by the action 

 of water and other reagents. 



It is such appearances that have been over and over again accepted as 

 proofs of the existence of cell-membranes, especially when, through a rent 

 in the cortical layer, the contents have been observed to protrude. And 

 in fact, this hardened peripheral layer of protoplasm does lead us on to 

 the cell-membrane as it becomes gradually more and more independent, 

 and assumes different chemical properties. 



But no one is able to define where this cortical layer of protoplasm 

 ends, and where the membrane of the cell begins a point essayed on all 

 sides in the case of animal cells at an earlier period of histological study. 



Occasionally, at some distance from the shrunken cell-body such a 

 covering with double contour may be recognised (fig. 49, d). But its 

 presence cannot be doubted for an instant, when, either mechanically, 

 as, for instance, by rupture and squeezing out of the contents, or by 

 chemical reagents which dissolve the latter, the membrane is successfully 

 isolated. Those fat-cells already mentioned (fig. 51, a) allow of the fluid 

 fat (b) being pressed out in drops, when such a membrane (c) becomes 

 recognisable. The same may be seen when the contents have been 



