156 GENERAL BIOLOGY 



the amount of reserve food-substance with which 

 it is packed. In spite of the disparity in size of the 

 two kinds of germ-cells, so far as the nucleus goes 

 both cells are very much alike. 



In animals, when a sperm-cell encounters an egg, 

 it penetrates the outer surface and generally insti- 

 gates some sort of a sudden change in the egg cyto- 

 plasm that prevents other sperms from entering. 

 In some forms, however (e.g. birds), it seems to be 

 the rule for a number of sperms to penetrate the 

 egg. But only one of them functions in the process 

 of zygosis. In most cases the tail is absorbed 

 and the nucleus moves in toward the center of the 

 egg-cell, while the egg-nucleus advances to meet 

 it, impelled by some sort of " attraction " not yet 

 understood. When the two nuclei, or " pro- 

 nuclei " as they are called at this time, have met, 

 they merge into one nucleus. The commingling 

 of chromatin material is probably not a true fusion, 

 inasmuch as the chromosomes into which the nu- 

 cleus is resolved appear to maintain their individ- 

 uality in subsequent cell divisions. From this fact 

 it results that, since the pro-nucleus of each gamete, 

 having undergone " reduction," has but half the 

 number of chromosomes characteristic of the species, 

 the nucleus of the zygote, composed as it is of the 

 sum of two such pro-nuclei, has the normal number 

 of chromosomes restored. In this way the chromo- 

 some number is maintained unchanged, from genera- 

 tion to generation. It must also be borne in mind 

 that since every cell of the organism traces its origin 



