1903 British Field Zoology 173 



was observed down to the 180th, when the strange sight was seen of 

 individuals of the same family attempting to conjugate with one another. 

 The results, however, were- ////, and the conjugates did not even recover 

 from the effects of their forlorn-hope. [See 'The Evolution of Sex,' by 

 Geddes and Thompson, p. 165.] It will be seen from the above that 

 in all animals the race deteriorates and " grows old " unless its 

 "rejuvenescence" is brought about by the introduction of new life 

 by the actual fusion of the protoplasm of two cells (entire individuals 

 in Protozoa — sex-cells in Metazoa), and that the best results follow 

 when the cells are not nearly related. 



Before quitting the subject of Amoeba we must mention a feature 

 which it has in common with all Protozoa, but which is found nowhere 

 else in the animal kingdom — the contractile vacuole (fig. 4 a). This 

 is a certain space in the body which is seen to slowly fill with clear 

 fluid (water ?) until it appears a globe within the animal, when it sud- 

 denly collapses and vanishes as though it had been pricked with a pin. 

 It again slowly fills and again collapses, repeating the process at regular 

 intervals. Various opinions have been advanced as to the function of 

 the contractile vacuole : some have thought that it might be an excretory 

 organ, analogous to the kidney; others that it is a primitive sort of heart, 

 propelling water through the body. 



We mentioned earlier in this chapter, when defining the cell, that every 

 cell has a denser body in the substance of the protoplasm, called the 

 nucleus. The shape of the nucleus varies greatly, especially amongst 

 the Protozoa, but in Amoeba it is globular or ovoid. It has been found 

 that whenever cell division takes place, the nucleus always divides first, 

 and this fact has a peculiar significance in the sexual reproduction of 

 higher animals — a subject beyond our present province. 



Cells which are identical in most respects with Amoeba are found 

 throughout the animal kingdom. They occur in certain parts of all 

 sponges : the corpuscles in the colourless blood of insects and crus- 

 taceans are amoeboid, and so are the colourless corpuscles found 

 amongst the red corpuscles in the blood of vertebrate animals. The 

 corpuscles of lymph are amoeboid, as also are the ova of mammals, &c, 

 in their very earliest unicellular stage. In one respect only do these 

 differ in structure from the typical Amoeba — they have no contractile 

 vacuole. The amoeboid cells of sponges, however, have a contractile 

 vacuole, which seems to indicate an affinity to the Protozoa. 



In speaking of the Amoeba's method of feeding we mentioned in- 

 cidentally that the food consisted of organic matter, and this illustrates 

 a striking physiological difference between animals and plants — viz., that 

 the former can assimilate only animal or vegetable matter, but the latter 

 are able to combine inorganic matter into organic compounds. The 

 only exceptions to this rule are Fungi, and some insect-eating plants, 

 which in this respect behave as animals. 



Amoeba, like other living organisms, requires a period of rest. On 

 the approach of winter or a droughty season the animal ceases to display 

 any vital activity and becomes enclosed in a leathery envelope or " cyst." 

 It is said then to be "encysted," and can remain so for an indefinite 



