REPRODUCTION AND LIFE CYCLES 417 



egg and in any event only one normally effects fertilization. The 

 head and middle pieces usually become separated from the tail piece 

 as penetration is effected, leaving the tail at the p(>rii)hcry of the 

 ovum in much the same way that sandals are left at the portal of a 

 Japanese house. The continued penetration of the remainder of the 

 sperm is made possible through movements of the cytoplasm within 

 the egg. The male element, which is now known as the male pro- 

 nucleus, absorbs water, enlarges, and finally becomes arranged on 

 the equatorial plate with the female -pronucleus of the ovum, and the 

 initial cell division follows. 



Results of Fertilization. The more important effects of ferti- 

 lization may be briefly summarized as follows : (1) Reproduction. 

 This is accomplished by restoring the normal (diploid) number of 

 chromosomes and by so doing producing a new center of cell division. 

 (2) Variation. As will be seen later, the whole phenomenon of 

 maturation of the germ cells and the consequent reduction of chromo- 

 somes to the haploid number makes possible new combinations and 

 variations between fertilized ova, or zygotes, upon which natural 

 selection may act. (3) Rejuvenescence. For years fertilization and 

 the concomitant stimulation of protoplasm have been thought neces- 

 sary to revivify an organism. Data have been collected both in 

 support of and in contradiction to this theory. Endomixis, as shown 

 by Woodruff (page 161), apparently acts as the rejuvenating agent in 

 nonconjugating strains of protozoa. 



Early Cleavage and Variations Caused by Yolk 



Once fertilization has occurred, cell division proceeds rapidly and 

 the zygote gives way to the early cleavage stages. In tlie simplest 

 types each plane of cleavage typically passes at right angles to the 

 preceding plane, the cells multiplying from the two-celled to the four- 

 celled stage, and so on up imtil the number in a given cleavage stage 

 cannot easily be determined. 



The amount of yolk present in the egg affects the cleavage rate and 

 even the pattern of development, since yolk is denser than typical 

 cytoplasm and, therefore, settles toward the lower side of the egg. 

 Its presence affects the rate of cell division by slowing it down. If 

 yolk is present in large amounts as in bird and reptile eggs, it tends 

 to occupy most of the available space in the ovum. In such ova 

 the embryo develops in the upper polar area, or in a restricted disk 

 called the blastoderm lying on top of the yolk mass. The ova of 



