196 COMPARATIVE ANATOMY 



trace of them can be seen. Whether the anal orifice is really 

 temporary, and its lips fuse together after an evacuation, or 

 whether it is permanent, and its lips are only pressed together, 

 is a matter of conjecture. 



The macronucleus and micronucleus lie just above the 

 mouth in the ectoplasm. The macronucleus is a relatively 

 large structure, measuring trom 30-35 /x in its longer, and 

 15-20 ju, in its shorter diameter. It has a definite nuclear 

 membrane which, in the resting condition, appears to be filled 

 with minutely granular contents staining deeply with the 

 ordinary dyes. The micronucleus, in the resting condition, is 

 a small ovoid body, measuring some 10 ft by 7 /*, placed 

 alongside of and close to the macronucleus. When at rest it 

 stains less intensely than the macronucleus, but its staining 

 capacity varies in the different phases which it goes through in 

 the course of division. 



There is good reason to suppose that the macronucleus is. 

 chiefly concerned with the nutritive, the micronucleus chiefly 

 with the reproductive processes of the animal, and the evidence 

 for this statement will become apparent during the description 

 of the processes of reproduction and conjugation. 



Reproduction in Paramecium is a simple process of trans- 

 verse binary fission. Both macronucleus and micronucleus 

 elongate prior to the division of the cell-body, and undergo 

 the modified form of mitosis shown in fig. 42. They first 

 become fusiform, and at either pole of the spindle polar plates 

 are formed connected by fibrils running the whole length of 

 the spindle. The spindle elongates, becomes dumb-bell 

 shaped, and finally the two swollen ends are connected by a 

 fine thread, which presently snaps in the middle, and the 

 nuclear division is complete. At the same time a constriction 

 divides the cell-body into two equal halves, which separate and 

 form two new Paramecia. A swarm of Paramecia, well 

 supplied with food, will continue to multiply by binary fission 

 for many generations. Careful observation has shown that the 

 divisions take place two or three times every twenty-four hours, 

 and that they will continue at this rate for some four or five 

 days. But not longer. Towards the close of this period the 

 offspring of a single Paramecium, some four thousand in 

 number, become smaller and begin to show signs of decay. 

 Eventually, if they are unable to conjugate, they undergo 



