220 INTRODUCTION TO CYTOLOGY 



. ment and redistribution of the nuclear substance without loss of any of its 

 essential constituents" (Wilson 1900, p. 233). 



In plants the discovery of reduction came somewhat later. Stras- 

 burger in 1888 showed that in angiosperms the number of chromosomes in 

 the egg and male nuclei is fixed by a reduction occurring in the mother- 

 cells of the embryo sac and pollen respectively. This was at once con- 

 firmed by Guignard (1889, 1891). E. Overton (1893) found that the 

 female gametophyte cells in the cycad, Ceratozamia, have half the number 

 of chromosomes found in the cells of the sporophyte. He further sug- 

 gested that reduction probably occurs in the sporocytes in mosses and ferns. 

 In the liverwort, Pallavicinia, Farmer (1894) found the gametophyte cells 

 to have four chromosomes and the sporophyte cells eight. That Overton's 

 theory of a reduction in the sporocytes of bryophytes and pteridophytes 

 was correct was demonstrated by Strasburger (1894), who postulated 

 the occurrence of a periodic reduction of the chromosomes in all organ- 

 isms reproducing sexually. 



The Stage in the Life Cycle at which Reduction Occurs. — The reduc- 

 tion of the chromosomes is accomplished during the course of two nuclear 

 divisions which, since in animals they have to do with the maturing of the 

 gametes, early came to be known as the maturation divisions. Because 

 of its peculiar character the first of these divisions was termed the hetero- 

 typic by Flemming (1887), while the second, which is essentially like a 

 somatic division, was called the homceotypic (sometimes written homo- 

 typic). Although the essential act of reduction usually occurs at the first 

 division, the entire process, to which the name meiosis has been applied, 

 is of such a nature that the second division is normally necessary for its 

 completion. As a result of the two divisions the " reduced " nuclei or cells 

 are formed in groups of four, or tetrads, though all members of a tetrad may 

 not function. The point in the life cycle at which these divisions take 

 place in various organisms will now be noted. 



In animals, almost without exception, reduction occurs at gameto- 

 genesis (Fig. 77). In the male those cells (spermatogonia) in the testes 

 whose ultimate descendants are to become spermatozoa multiply by 

 divisions of the ordinary equational type until a certain number are 

 produced. These cells, now called primary spermatocytes, enlarge a little 

 and quickly undergo two successive divisions: the first division in each 

 is heterotypic and results in two cells called secondary spermatocytes; the 

 second is homceotypic and divides the two secondary spermatocytes into 

 four spermatids, each of which becomes transformed into a spermatozoon. 

 The four spermatozoa are therefore the immediate result of the two 

 maturation divisions. In the female the situation is somewhat different: 

 here nearly all of the differentiation of the gamete is accomplished before 

 the nuclear divisions bringing about reduction actually occur. The 

 primary oocytes (ovocytes) are the descendants of a number of generations 



