224 COMPARATIVE ANATOMY 



and solid excretory substances to the exterior. This is fur- 

 nished by the so-called temporary anus or cytoproct, a spot 

 situated a little way behind the mouth opening on the ventral 

 surface. The cytoproct is not a permanent aperture, or at 

 least it is not permanently open, and it can only be detected 

 at the moment when excreta are being discharged through it. 

 Then it has the appearance of a minute circular orifice placed 

 upon a small papillary projection of the body. But as soon 

 as the solid matter is voided the aperture and papilla vanish, 

 and no trace of them can been seen. Whether the 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 meganucleus and micronucleus lie just above the 

 mouth in the ectoplasm. The meganucleus is a relatively 

 large structure, measuring from 30-35 p in its longer, and 

 15-20 /x 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 ordin- 

 ary dyes. The micronucleus, in the resting condition, is a 

 small ovoid body, measuring some to 10 ^ by 7 /*, placed along- 

 side of and close to the meganucleus. When at rest it stains 

 less intensely than the meganucleus, 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 meganucleus is 

 cniefly 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. Prior to the division of the cell-body 

 both meganucleus and micronucleus elongate and divide, the 

 former amitotically, the latter by a simple form of mitosis. 

 In either case the nucleus becomes dumbbell-shaped, the 

 chromatin passes to its extremities, until 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 in the shape of fresh hay 



