68 AN INTRODUCTION TO ZOOLOGY 



furrow becomes deeper and deeper until only a slender thread of 

 protoplasm holds the two halves of the body together. This 

 connection is finally severed and the two daughter Paramecia 

 are freed from each other. Each contains both macro- and micro- 

 nuclei, two contractile vacuoles, and a mouth with gullet. The 

 entire process occupies about two hours. The time, however, 

 varies considerably, depending upon the temperature of the water, 

 the quality and quantity of food, and probably other factors. 

 The daughter Paramecia increase rapidly in size, and at the end of 

 twenty-four hours divide again if the temperature remains at 

 from 15 to 17 C. ; if the temperature is raised to i7-2o C., 

 two divisions may take place in one day (83). 



Conjugation. At a certain time in the life cycle of Paramecium 

 conjugation occurs. The conditions that initiate this process are 

 not yet known, but the complicated stages have been quite fully 

 worked out. When two Paramecia, which are ready to conjugate, 

 come together, they remain attached to each other because of the 

 adhesive state of the external protoplasm. The ventral surfaces 

 of the two animals are opposed, and a protoplasmic bridge is 

 constructed between them. As soon as this union is effected, the 

 nuclei pass through a series of stages which have been likened to 

 the maturation processes of metazoan eggs (Chap. VII, p. 103). 

 Reference to Figure 31 will help to make clear the following de- 

 scription. The micronucleus moves from its normal position in 

 a concavity of the macronucleus (Fig. 23, mi.), and grows larger, 

 its chromatin breaking up into granules which radiate from a 

 division center at one end (Fig. 31,0). The nucleus then length- 

 ens, forming a spindle, and subsequently divides into two (b). 

 These immediately divide again without the intervention of a 

 resting stage. The resultant four nuclei (c) have been compared 

 to the four sperms produced by a primary spermatocyte or to an 

 egg with its polar bodies, and the divisions are considered as the 

 first and second maturation mitoses (see pp. 103-108). Three 

 of the four nuclei degenerate (d), the fourth divides again. 

 During this division there are no definite spindle fibers and no 



