18 COMPAEATIVE ANATOMY. 



in the protoplasm is essential to another kind of pliEenomena, to 

 wliicli we now pass. Tlie substances contained in tlie protoplasm 

 become separated from it^ that is, are secreted from it. This 

 process of secretion varies in character; it sometimes occurs within 

 the protoplasmic body, and then substances differing in their chemico- 

 physical properties from the protoplasm are formed within the cell. 

 These substances may be distinguished by their constitution, such as 

 fat, colouring matter, and so on; or by form, such as granules, drops, 

 crystals. Sometimes secretion affects the surface of the protoplasm ; 

 and then the secreted substance may be fluid, in which case it will 

 get separated and removed from the protoplasm; or it may be 

 solid, in which case it will remain more or less intimately connected 

 to the rest of the unaltered protoplasm. Substances which are 

 different from the rest of the protoplasm of a cell arise by chemico- 

 physical changes of the whole sui-face, or of a part of it. We 

 may regard these changes in the protoplasm as differentiations, 

 for they are differentiated from matter which was previously in 

 an indifferent condition within the protoplasm. In this way the 

 structure which has been already alluded to as the cell-membrane, 

 is formed at the periphery of the cell. But this same process leads 

 to other arrangements also, which we shall have to examine more 

 closely hereafter. 



The series of vital processes exhibited by a single cell essentially 

 agrees with those exhibited by any and every other organism, so 

 that the cell itself is virtually an organism (Elementary organism). 



Differentiation of the Animal Organism. 



§ 16. 



The simplest and lowest stage of the animal organism is repre- 

 sented by the earliest stage of its development, in which it is 

 known as the egg. Except in some exceptional cases, which only 

 prove the rule, and which need not be mentioned hei'e, this egg 



is nothing more nor less than a cell. The 

 egg-cell does not differ from other cells in 

 any essential points; it may increase in 

 size, and special particles — yolk granules — 

 may appear in its protoplasm. Although the 

 presence of these latter alters the character 

 of egg-ceUs as indifferent cells, yet it 

 does not destroy their character as cells, 

 which is just as little affected by this as in 

 Fig. 1. Diagram of other cells by the differentiation within their 

 P^otoprm. "b^NTcS: ^«<iies Of substances, such as chlorophyll 

 (Germinal vesicle). cNu- granules, starch, pigment granules, &c. ihis 

 cleolus (Germinal spot) . condition of the egg-cell, which on the whole 



