OKIGIN OF CELLS. 15 



uninterrupted series of generations of cells, each cell becoming so modi- 

 fied as to specialize certain functions which are together possessed by all 

 forms of undifferentiated protoplasm. Thus, in the higher forms certain 

 cells will be found to have become so modified as to have the function of 

 reproduction especially developed ; they will therefore constitute the 

 reproductive tissues. In other cells the nutritive functions will become 

 most prominent ; they will therefore form part of the tissues whose 

 function is to preserve the nutritive balance of the organism. Specializa- 

 tion of function is therefore the explanation of the development of 

 tissues; the result is a plr^siological division of labor. We will have to 

 return to this subject again. 



The germ of every animal and vegetable organism is a cell which 

 owes its existence to some similar, previously-existing cell. Neglecting 

 the origin and development of cells in the vegetable kingdom, every 

 cell which forms part of the organs or tissues of all forms of animal 

 life originated in and developed from a germ-cell or ovum. 



The ovum of man and other mammals is 

 a minute mass of protoplasm, corresponding in 

 its general appearance with the description of a 

 typical cell. The protoplasm, or cell-contents, 

 is surrounded by a delicate, striated membrane, 

 the Zona radiata, or vitelline membrane. Within 

 the cell-contents, in addition to numerous minute 

 particles, the so-called yelk- globules, is a 

 collection of denser particles of protoplasm, FIG. 3. TYPICAL ANIMAL 

 the nucleus, or germinal vesicle, and within uS?"cxjJ) VUM OF 

 that, again, one or more still more solid masses, A 

 the nucleoli, or germinal spots. 



The cell-contents is identical in nature and properties with the sub- 

 stance of the amoaba, and before and immediately after fertilization may 

 even be the seat of spontaneous movements of contraction and expan- 

 sion. When mature, its diameter in the domestic animals and man varies 

 from the ^J^ to the T ^ of an inch (0.18-0.2 mm.). 



Fertilization leads to a cleavage of the protoplasm into two parts, 

 the nucleus being first divided, so that two new elements originate from 

 the ovum, each consisting of protoplasm and each containing a nucleus. 

 Each of the two new cells thus formed again subdivide into four, these 

 into eight, and so on through many generations, until a large number of 

 new cells, the so-called "mulberry mass," results from the subdivision of 

 the original parent cell. 



According to Schleiden and Schwann, the founders of the cell doc- 

 trine, organic forms of life may originate in one of two wa} r s, either by 

 the aggregation of granules in a previously-existing homogeneous fluid 



