AN INTRODUCTION TO THE METAZOA 



8l 



cells which, after fertilization, grow into budding individuals. 

 There is here an alternation of an asexual budding generation 

 with a sexual generation. Obelia, as will be explained later 

 (Fig. 73), develops metagenetically. 



(2) The Origin of the Egg and Spermatozoon. — Spermato- 

 genesis. — The origin of the male germ-cell or spermatozoon 

 is termed spermatogenesis. As shown in Figure 47, this process 

 may be divided into three periods: (a) the multiplication of 



PRIMORDIAL 

 GERM-CELL 



SPERMATOGONIA, 



PRIMARY / • J 



SPERMATOCYTE 



SECONDARY 

 SPERMATOCYTES 



SPERMATIDS 



SPERMATOZOA 



MULTIPLICATION 

 PERIOD 



MATURATION 

 PERIOD 



Fig. 47. — Diagram illustrating the stages of spermatogenesis. The primordial 

 germ-cell is represented as possessing four chromosomes. 



the primordial germ-cells or spermatogonia, (b) the growth 

 of these cells, and (c) their ripening or maturation. These 

 stages occur in all Metazoa from the lowest to man. 



No one knows how many cells are produced during "the period 

 of multiplication. The last generation of spermatogonia gives 

 rise by division to the primary spermatocytes. The latter increase 

 greatly in size during the long growth period, and in each of 

 them the chromosomes unite or conjugate to form double or 

 bivalent chromosomes. Each primary spermatocyte gives rise 

 by division to two secondary spermatocytes. The secondary 

 spermatocytes immediately divide, each forming two spermatids. 



