36 ANIMAL BIOLOGY 



Amoeba, under favorable conditions, grows rapidly and, when 

 it has attained the size limit characteristic of the species, cell 

 division, termed binary fission, takes place, with the result that 

 from the single large cell there are formed two smaller individuals 

 which soon become complete in all respects. These, in turn, grow 

 and repeat the process so that, as in the case of Protococcus, 

 within a few days the original Amoeba has divided its individual- 

 ity, so to speak, among a multitude of descendants. (Fig. 8.) 



Clearly Amoeba performs all the essential vital functions that 

 become an animal. Such being so, it is important to compare the 

 metabolism of Amoeba, the animal, with that of Protococcus, 

 the green plant. 



2. Food Taking 



The food of Amoeba is chiefly other microscopic animals and 

 plants that it meets in its environment of pond water or vegetable 

 infusion. Coming in contact with its prey, pseudopodia are extended 

 about it and soon the prospective food material is enclosed with 

 the endoplasm of its captor. Here the food is surrounded by a 

 droplet of fluid, a gastric vacuole, into which the endoplasm se- 

 cretes chemical substances (enzymes, etc.) which gradually simplify 

 — digest — the complex proteins, carbohydrates, etc., of the food. 

 Finally, this material, which shortly before was the protoplasm of 

 another organism, is incorporated into Amoeba protoplasm — 

 matter and energy is supplied and the animal lives and grows. 



This is, in most respects, a strikingly different condition from 

 that which we have seen in Protococcus. In Amoeba solid par- 

 ticles of food — tiny animals and plants — are taken into the 

 cell, and since the chief organic constituents of protoplasm are 

 proteins, associated with carbohydrates and fats, it is clear that 

 the income of the animal organism is, unlike that of the green 

 plant, chiefly ready-made complex foodstuffs. In other words, 

 Amoeba, like all animals, requires relatively complex chemical 

 compounds rich in potential energy: proteins, carbohydrates, and 

 fats. Of these, proteins or their constituent amino acids are ab- 

 solutely indispensable because it is only from this source that 

 nitrogen is available for the animal. But the green plant, through 

 its chlorophyll apparatus, is able to take materials largely devoid 

 of energy and to rearrange them and endow them with potential 

 energy which it has received in the kinetic form from sunlight. 



