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PARAMECIUM 67 



and finally downward and along the ventral surface toward the 

 mouth (indicated by arrows in Fig. 23). During this journey 

 digestion takes place. 



Unlike Ameba a special anal spot (Fig. 23, an.) is present in 

 Paramecium through which indigestible solids are discharged to 

 the outside. This opens on the ventral surface just behind the 

 mouth. It can be seen only when material is cast out. It is not 

 yet known whether the anal spot is a permanent orifice whose 

 lips are so tightly closed as to be invisible to us or whether a fresh 

 opening is made at each discharge. The processes of digestion, 

 absorption, dissimilation, excretion, respiration, and growth are 

 so similar to those described for Ameba that they need not be 

 considered further at this place (see pp. 49-50). 



Reproduction. Paramecium reproduces only by simple 

 binary division. This process is interrupted occasionally by a 

 temporary union (conjugation) of two individuals and a subse- 

 quent mutual fertilization. 



Binary fission. In binary fission the animal divides trans- 

 versely (Fig. 30). The first indication of a forthcoming division 

 is seen in the micronucleus, which undergoes a sort of mitosis 

 (Fig. 30, 8 and ici), its substance being equally divided between 

 the two daughter nuclei; these separate and finally come to lie 

 one near either end of the body. Figure 30 shows two dividing 

 micronuclei, since there are two of these in Paramecium aurelia. 

 The macronucleus elongates and then divides transversely 

 (Fig. 30, 6). The gullet produces a bud which develops into 

 another gullet; these two structures move apart, the old gullet 

 advancing to the ventral middle line of the forepart of the body, 

 and the new one to a similar position in the posterior half 

 (Fig. 30, 7 and 9). The undulating membrane remains with the 

 old gullet while a new one arises in connection with the new gullet. 



new contractile vacuole (Fig. 30, i) arises near the anterior end 

 of the body, another just back of the middle line (Fig. 30, 4). 

 While these events are taking place a constriction appears near 

 the middle of the longitudinal diameter of the body; this cleavage 



