THE PROTOZOA 



129 



cytoplasm becomes loaded up with dark granules ; in this surfeited 

 condition the animal becomes very sluggish and unable to continue 

 its ordinary activities, and we say it is in a state of depression. If a 

 small quantity of certain Potassium salts be added to the water the 

 dense coloration gradually disappears and normal activities are 

 resumed. It is noteworthy that the protoplasm first starts to get 

 clear in the neighbourhood of the macronucleus, whence it spreads 

 outwards, indicating that this body is the 

 centre of the chemical changes that have 

 taken place in the cell. When all the 

 digestible part of the food has been as- 

 similated the residue is egested via the 

 anal spot towards the posterior end of 

 the animal. 



Respiration occurs all over the general 

 surface of the body, and the function of 

 nitrogenous excretion is carried out by the 

 pulsating vacuoles which discharge a fluid 

 containing uric acid in some form ; both 

 are very like the same processes in Amceba. 

 Reproduction is also somewhat 

 similar, and takes place by means of 

 binary fission, the plane of division lying 

 transversely to the long axis of the ani- 

 mal. Just prior to the division of the 

 cytoplasm, division of the nuclei occurs. 

 The meganucleus elongates, becomes 

 dumb-bell shaped, and then separates into 

 two parts, one travelling to each end of 

 the body in a way quite typical of direct 

 nuclear fission. The micronucleus divides 

 slightly later than the meganucleus, and 

 by a form of indirect division. It first 

 enlarges, and the chromatin within it 

 after passing through a reticular or net- 

 like stage breaks up into a large number of definite small rod- 

 shaped bodies known as the chromosomes, which pass towards the 

 middle of the nucleus. At each end of the nucleus, by this time some- 

 what elongated, appears a modified protoplasmic area derived from 

 the division of a sort of nucleolus-like body termed the nucleo- 

 centrosome. The areas, from their position at the poles of the 

 nucleus, are called the polar plates, and they appear to play an 

 important part in the processes connected with division. Between 

 them appears a number of fine fibrillae forming the spindle, in the 



K 



FIG. 41. Diagram of a 

 longitudinal section of 

 a dividing Paramcecium, 

 adapted from a photo- 

 graph of Calkins. 



C., cilia ; F.V., food vacuole ; 

 M. and M 1 ., divided macronu- 

 clei ; Mi., micronucleus ; T., 

 exploded trichocysts. 



