16 



DIVISION OF METAZOAN NUCLEUS 



creases in size and becomes the prmiary spermatocyte. By division two 

 secondary spermatocytes are produced, and each of these again divides, 

 giving rise to four spermatids, which become directly transformed into 

 microgametes or spermatozoa. It is during the first or second of these 

 two divisions that meiosis occurs and the number of chromosomes is 

 reduced. When it occurs it is seen that, as the chromosomes arrange 



Fig. 4. — Diagram of Meiosis or Reducing Division of a Nucleus with Four 

 Chromosomes. (Original.) 



A. Showing two pairs (dotted and lined) of homologous chromosomes and commencing formation 



of spindle. 

 B and C. Syndesisor conjugation of homologous chromosomes. 

 D and E. Separation of the conjugated homologous chromosomes. 

 F. Formation of nuclei, each with half the original number of chromosomes; one of each pair of 



homologous chromosomes has entered each nucleus. 

 In ordinary mitosis the chromosomes at C, instead of separating, would divide, so that two pairs 



of homologous chromosomes would pass to each daughter nucleus. 



themselves at the equator of the spindle, the individuals of each pair 

 of homologous chromosomes are closely applied to one another, so that 

 at first inspection it might be thought that only half the number were 

 present. This approximation of the chromosomes of each pair is known 

 as the conjugation of the chromosomes or syndesis, and it is supposed that 



