MATURATION 323 



the reduction division, because it leaves the nucleus with half 

 the normal number of chromosomes. When now fertilization 

 takes place the sperm nucleus and the egg pro-nucleus fuse, and 

 there results therefrom a new egg nucleus with the normal num- 

 ber of chromosomes. In many ways minor variations occur, 

 but the essential features seem to hold throughout the animal 

 and vegetable kingdoms. Only one variation will be described. 



699. Conjugation in Potozoa.— In many protozoa conjuga- 

 tion occurs in a peculiar form. The cells do not fuse and only 

 the nucleus is greatly affected by the process. In Paramoecium, 

 for example, two individuals adhere to each other by their oral 

 surfaces and remain in this condition for some time. Finally, 

 they separate, the same two individuals, at least in external ap- 

 pearance. However, radical changes have occurred in the nuclei, 

 which are briefly as follows: The macronucleus disintegrates 

 and finally disappears without apparently having taken any 

 part in the process of conjugation. The micronucleus forms a 

 spindle and divides. This is repeated so that four daughter 

 nuclei are produced. Three of these also disintegrate while the 

 fourth divides again. There are now two active nuclei in each 

 cell, and one of these passes out of one cell to the other through 

 the cell mouths, which are placed one over the other. These 

 nuclei are regarded as the equivalents of sperm nucleus and egg 

 pro-nucleus, and each is said to have approximately half the 

 usual chromatic substance. From the fusion of the two nuclei, 

 the receptive nucleus and the wandering nucleus, a new nucleus 

 is produced, from which the new macronucleus and the new 

 micronucleus are both developed. 



700. Fertilization. — The approach of the sperm to the egg 

 cells is not a matter of accident. Observation of the movement 

 of the active sperm in the presence of a ripe egg is sufficient to 

 persuade one that there is a positive stimulus which directs them 

 to the egg. In a short time large numbers are swarming about 

 the egg and apparently endeavoring to penetrate its surface. 



