284 THE COMPLETE GRAZIER. BOOK n. 



it by the blood, for every alveolus in the mammary gland is surrounded 

 by a dense network of blood capillaries, and the exceedingly thin walls 

 of these as well as of the secreting cells are easily permeable by fluid. 

 As has already been indicated, the blood is dependent for its nourish- 

 ment upon the food. 



Reverting now to the formation of fat by the secreting cells of the 

 mammary gland, the mechanism of this process may perhaps be better 

 understood by studying analogous, though not strictly similar, processes 

 which are observed to occur, firstly, in a certain low form of animal life, 

 named Amoeba, and, secondly, in the epithelial cells of the intestinal 

 villi during the absorption or appropriation of fat. Amoeba is a minute 

 microscopic organism which may be found in stagnant water, or in 

 aqueous infusions of animal matter. It has very much the appearance 

 of a particle of jelly, it consists almost entirely of protoplasm, and is 

 continually changing its form. A nucleus is often present. The 

 gelatinous body of the amoeba can hardly be regarded as possessing a 

 distinct external covering, though the circumferential part does differ 

 in some minor details from the deeper-lying portions. This limitary 

 layer has been aptly compared to the wall of a soap-bubble, which, 

 though fluid, has a certain cohesion which not only enables its particles 

 to hold together and form a continuous sheet, but permits a rod to be 

 passed into or through the bubble without bursting it ; the walls closing 

 together, and recovering their continuity as soon as the rod is drawn 

 away. In a similar way the amoeba feeds, taking in and passing out 

 solid matter, though the animal possesses no aperture ; the solid body 

 passes through the outer wall, which immediately closes up and repairs 

 the rent. Thus does the amoeba take in the small, usually vegetable, 

 organisms, which serve it as food, and subsequently get rid of the un- 

 digested solid parts. 1 From the food thus obtained the organism can 

 make new protoplasm, and produce other organisms like itself. It 

 lives, moves, eats, grows, and after a time dies, having been during its 

 whole life hardly anything more than a minute lump of protoplasm. 

 Certain substances serving as food are received into the body of the 

 amoeba, and there in large measure dissolved. The dissolved portions 

 are subsequently converted from dead food into new living protoplasm, 

 and become part and parcel of the substance of the organism. Simul- 

 taneously there is going on an ejection of old material, for the proto- 

 plasm is incessantly undergoing chemical change (metabolism), room 

 being made for the new protoplasm by the breaking up of the old 

 protoplasm into products which are cast out of the body and got rid of. 

 These products of metabolic action have, in many cases at all events, 

 subsidiary uses. Some probably serve to dissolve the raw food intro- 

 duced into the amoeba, and remain in its body for some little time for 

 this purpose. Such products are generally called secretions ; others 

 which pass more rapidly away are spoken of as excretions. The dis- 

 tinction between the two is unimportant and frequently accidental. 2 



In the case of the intestinal villi, it is absolutely certain that the 



1 Huxley, "Practical Biology." 

 a Foster, "Physiology." 



