174 CHEMICAL NATURE OF PROTOPLASM. 



they join one another they may be slightly enlarged. These enlargements at the 

 junctions of the fibrils as well as the optical sections of those fibrils which are coursing 

 in a direction more or less parallel to the visual axis of the observer appear, when 

 the horizontally running fibrils are not clearly seen, as distinct points or granules in the 

 protoplasm ; it is perhaps for this reason that the cell-substance was formerly con- 

 stantly described as being finely granular. 



The external layer of the protoplasm of many cells, especially that of fixed cells 

 like those which constitute the varieties of epithelium, and in a notable degree the 

 cells of the higher plants, may become altered by the deposition in or upon it of 

 chemical substances which render it firmer and more resistant than the rest of the 

 cell. In this way a cell-membrane (fig. 203, me) may be formed, either complete or, 

 more commonly, furnished with minute pores which allow of the passage of fluid or 

 the connection of the cell with its neighbours by filaments of protoplasm which 

 traverse the pores. In those cases where a cell-membrane is formed the peripheral layer 

 of the protoplasm is usually furnished with a denser reticulum than the rest, but this 

 is not the case in cells like the white corpuscles of the blood and the amoeba which have 

 no cell-membrane. In these the exterior of the protoplasm may be clearer than the 

 interior, indeed a reticulated appearance may be altogether wanting in it, or may be re- 

 placed by a few radial striae. Such clearer peripheral protoplasm is sometimes known as 

 the " ectoplasm " to distinguish it from the granular-looking " endoplasm " around 

 the nucleus. 



Chemical nature of protoplasm. Protoplasm may contain imbedded in its 

 substance (generally in the hyaloplasm) various adventitious materials, such as 

 granules of proteid or starch, globules of fat, crystals of various kinds, pigment- 

 granules and globules of watery fluid (vacuoles). These may have been formed 

 by the cell either from its own substance or directly from materials imbibed in a 

 fluid form, or they may have been bodily incepted in the solid form, but they are not 

 essential to its composition, although no doubt when they occur they play an 

 important part in relation to its vital activities. 1 Apart from these the protoplasm 

 of all vegetable and animal cells, since it is similar in its functional manifestations, 

 is also in all probability similar in chemical constitution. So far as the 

 proteids are concerned, it would appear that there is a preponderance of the less 

 stable members of that group, such as globulins and albumoses as distinguished 

 from the more stable albumins, and that these cell-proteids appear to be associated 

 with ferment-like reactions like those which produce digestive or coagulative 

 changes. The chief proteid found is a muco-globulin, containing phosphorus, to 

 which the name plastin has been applied. It would further seem that the presence 

 of certain inorganic substances, and especially calcium, is essential to the 

 life and therefore to the functions of protoplasm, but in what manner the 

 lime may be combined with the organic basis of the living material, remains 

 as yet quite undetermined. It must further not be forgotten that water, enters 

 very largely into the composition of all living material. 2 



CHEMICAL AND PHYSICAL CHANGES. AMOEBOID MOVEMENTS. 



Vital phenomena of protoplasm. During the life of a cell its protoplasm' is 

 constantly undergoing chemical and physical changes. The chemical changes are in 

 some measure determinable by comparing the products which are given off by the 



1 The terms " deutoplasm " and " paraplasm " (Kupffer) have sometimes been applied to materials 

 contained within a cell, which are not considered to constitute a part of the actual protoplasm. 



2 For an account of the present state of knowledge regarding the chemistry of protoplasm and of the 

 cell generally, consult Halliburton, "A Textbook of Chemical Physiology," 1891, pp. 190 to 211. 



