138 COMPARATIVE ANATOMY 



characteristic vital phenomena of irritability, automatism, 

 assimilation, and excretion. We cannot doubt that it is also 

 metabolic. The pseudopodial movements are manifestations 

 of energy ; they imply waste of material, and we can infer from 

 the fact that the animalcule takes in and digests food that 

 the waste of its substance is made good by the elaborated 

 products of digestion. In so small an organism the actual 

 processes of waste and repair cannot be followed out in 

 detail, none the less we can assert that they are performed. 

 So, too, we can assert that the Amoeba is respiratory, that the 

 energy exhibited in pseudopodial movement is the result of 

 the oxidation of the tissues, and that one of the waste products 

 is carbon dioxide. There is no definite respiratory organ, 

 unless, indeed, the contractile vacuole functions as such, and 

 the amount of carbonic acid gas given off is too small to be 

 collected and measured. But the same observation which 

 showed that Pelomyxa is sensitive to light shows also that it 

 requires oxygen. 



Lastly, Amceba is reproductive. Under normal conditions, 

 when food is abundant and the temperature sufficiently high, 

 it propagates its kind by the simple method of binary division, 

 but under less favourable circumstances by a more complicated 

 process known as encystment with spore formation. It is 

 difficult, however, to give a precise account of the life-cycle of 

 any one species of Amoeba, for it is a remarkable fact that, 

 although it is one of the commonest objects of study, the 

 reproductive processes of this animalcule have been but rarely 

 seen and described. It is even now uncertain whether mitotic 

 or amitotic division of the nucleus during binary division is 

 the more common. It is at all events certain that amitotic 

 division does occur. In Amceba crystalligera, the nucleus 

 consists of a central mass or kernel, which stains but feebly 

 with ordinary dyes and has been considered as the equivalent 

 of a nucleolus. Around this kernel is an envelope of material 

 which stains deeply and is undoubtedly chromatin. Outside 

 this again is a cortical layer which scarcely stains at all. The 

 characteristic alveolar structure of protoplasm is distinguishable 

 in all three components of the nucleus (fig. 29, E). When 

 division is about to take place the nucleus, previously spherical, 

 becomes oval and is eventually drawn out into an elongated 

 dumb-bell shape (fig. 29, C and ,), all three layers sharing in 



