14 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



are concerned, cannot be distinguished from the contents of all active 

 forms of cells (Fig. 2). 



The amoeba is capable of spontaneous motion, both as regards 

 change of external form and of progressing from place to place. 

 Motions may also be evoked by various stimuli ; hence, free protoplasm, 

 in common with muscular fibre and ciliated organisms, is contractile. 

 The peculiarity of protoplasmic motion, as seen in the amoeba, is that 

 motion does not occur around a fixed point, but rather is a flowing 

 motion, such as might occur in the particles of a fluid. Thus, in an 

 amoeba the changes in form and location are effected through the thrust- 

 ing out of lobe-like prolongations of the periphery (pseudopodia), and 

 their subsequent withdrawal or the flowing into these extensions of the 

 remainder of the body. 



Occasionally one or more of these pseudopodia become gradually 

 more and more constricted, until, finally, a portion becomes entirely 



separated from the original 

 mass, increases in size, and 

 itself possesses all the proper- 

 ties of the parent stock; hence, 

 protoplasm is reproductive, 

 and possesses the power of 

 growth. Moreover, the move- 

 ments of an amceba are not 

 necessarily the consequences 

 of external stimuli, but may 

 be self-originating; hence, 

 protoplasm is also automatic. 

 If watched for some time, an amceba will often be seen to take into 

 its interior, by flowing around them, small vegetable organisms, of which 

 portions are dissolved and converted into the substance of its body, 

 while the undigested remainder is extruded ; therefore, protoplasm, even 

 in the absence of all digestive organs, possesses the power of nutrition. 

 The amoeba requires for its existence an atmosphere of oxygen, 

 which is absorbed, and which it again partly exhales as carbon dioxide. 

 Protoplasm is therefore respiratory. 



Fig. 2. — A Non-Nucleated Cell, the Pkot- 



AMffiBA PKIMITIVA, AFTEB HAECKEL. (WUTUtt.) 



A, original condition ; B, commencement of reproduction by fission ; 

 C, after complete separation. 



II. ORIGIN OF CELLS. 



We have seen that in the amceba a simple mass of undifferentiated 

 protoplasm. possesses the powers of reproduction, contractility, respira- 

 tion, irritability, nutrition, and automatism. Every form of life com- 

 mences its existence in the form of just such a simple mass of proto- 

 plasm. Starting with the ovum,' and ending with the nucleated elements 

 found in the organs and tissues of the embryo and adult, there is one 



