198 COMPARATIVE ANATOMY 



usual length of Paramecium caudatum is 300-325 //., the 

 gametes rarely exceed 210 p in length. As a rule the process 

 of conjugation begins during the late hours of the night and 

 the early hours of the morning, and it lasts till late in the 

 following afternoon. The first consequence of conjugation is 

 that the micronucleus of each gamete goes through what may 

 be called the first stage of its evolution. The result of which 

 is that its diameter is doubled. Before conjugation it measured 

 some 10 \i x 7 p. After a period of 4-5 hours it measures 

 20-24 P x 12-14 p. During this time it has undergone 

 considerable changes of shape, becoming first elongate, then 

 spindle-shaped, and then doubled up to form a crescent, whilst 

 its chromatin granules are aggregated in rows to form longitu- 

 dinal fibrils. But eventually it resumes its original ovoid 

 condition. This period of growth is distinguished as stage A, 

 and is represented in the annexed diagram (fig. 43) by the 

 space enclosed between the two lowest horizontal lines. The 

 two next stages, B and C, are occupied by the division of the 

 micronucleus. This structure, increased as it is to twice its 

 former diameter and eight times its original volume, passes 

 through the mitosis represented in fig. 42 and divides into two, 

 and each product of division immediately undergoes mitosis 

 and again divides, so that there are now four micronuclei. The 

 macronucleus meanwhile has undergone no change. The four 

 micronuclei in each gamete are of equal size and to all appear- 

 ances equivalent, and all of these begin to prepare for a new 

 division by elongating to form fibrous spindles ; but only one, 

 and it is always the micronucleus which happens to be nearest 

 to the mouth, passes through the further stages of mitosis and 

 divides. The three others are arrested at the spindle stage, and 

 then gradually degenerate and are absorbed, leaving not a trace 

 behind. The surviving micronucleus in each gamete completes 

 its division so that at the close of stage D each gamete has two 

 micronuclei and one as yet unaltered macronucleus. Though 

 these micronuclei are quite similar to one another and, except 

 for their size, to the micronuclei which preceded them, they take 

 up definite positions which would seem to determine their 

 future fate. In each gamete one of the micronuclei is placed 

 close against the mouth, and may be called, in anticipation, the 

 male,, whilst the other is the female pronucleus. Both male 

 and female pronuclei elongate and form fibrillated spindles, 



