SYNGAMY AND SEX IN THE PROTOZOA 143 



share in syngamic processes, and the vegetative chromatin, whether 

 as chromidia or a nucleus, disappears from the life-history. 



Nuclear reduction, in the strict sense, concerns simply the nuclei 

 composed of generative chromatin, and is a process which results 

 in the reduction of the chromatin to half the specific quantity, a 

 deficiency made up again to the full amount by the union of the 

 two pronuclei to form the synkaryon. It is therefore a process 

 which is seen in its most characteristic form in those cases where 

 it is possible to gauge the amount of chromatin in the nucleus 

 more or less accurately by the number of chromosomes formed 

 during division. 



In the Metazoa, where each species is characterized by possessing 

 a number of chromosomes which is generally constant (the so-called 

 " somatic number "), the process of reduction appears to be ex- 

 tremely uniform in its essential details throughout the whole series, 

 from the Sponges and Ccelenterates up to man, and admits of a 

 description in general terms. The gametocyte (oocyte or sperma- 

 tocyte), when at the full term of its growth, has a large nucleus 

 which then goes through two maturative divisions in rapid succes- 

 sion. When the gametocyte-nucleus prepares for division, it 

 appears with half the somatic number of chromosomes ; but each 

 chromosome is in reality bivalent, and produced by the fusion or 

 close adherence of two separate somatic chromosomes. In the 

 first reduction- mitosis, the two adherent chromosomes in each case 

 separate from one another and travel to opposite poles of the 

 spindle ; hence this division is in reality a reducing, though it 

 simulates in some of its features an equating, division. Im- 

 mediately or very soon after the two chromosomes of each pair 

 have separated, they split longitudinally in preparation for the next 

 mitosis, which follows hard upon the first, and in which the two 

 sister-chromosomes of each pair go to opposite poles of the spindle. 

 Consequently the second reduction-division is in reality an equating 

 mitosis, though on account of the precocious splitting of the chromo- 

 somes it may simulate a reducing division. Thus, to sum up the 

 process briefly, the number of chromosomes in the germ-cells is 

 reduced to half the somatic number by two successive mitoses, 

 the first a reducing, the second an equating division. In the male 

 sex, the spermatocyte divides into four gamete-cells of equal size, 

 the spermatids, each of which becomes a spermatozoon. In the 

 female sex the oocyte-divisions are very unequal, producing the 

 ovum, ripe for fertilization, and three minute sister-cells of the 

 ovum which, as the so-called " polar bodies," are cast off and die 

 away. By syngamy between a ripe ovum and a spermatozoon, 

 each containing half the somatic number of chromosomes, the full 

 somatic number is restored. 



