REPRODUCTION AND DEVELOPMENT IN CHORDATES 



from the division of the primary spermatocyte are secondary spermatocytes; 

 they begin the second meiotic or second maturation division almost immediately. 

 A spindle is formed, and the sister half-chromosomes (dyads) become arranged 

 at the equatorial plate. After replication of the kinetochore, separation occurs 

 during the anaphase in such a way that the two chromatids of each chromo- 

 some pass to opposite poles of the spindle. This is the equational division and 

 is entirely comparable with any mitotic division, except that only half the 

 typical number of chromosomes is present in the cell. The two cells formed 

 by the division of each secondary spermatocyte are the spermatids. Four 

 spermatids arise from each primary spermatocyte, and each one contains one 

 chromatid of the four which are formed by duplication of the members of each 

 pair of homologous chromosomes. 



Cytosomal differentiation of the spermatids occurs next; this is known as 

 spermiogenesis (Fig. 5.8). The essential changes consist in the outgrowth of 

 a vibratile flagellum from one of the centrioles of the spermatid and the loss 

 of most of the cytoplasm. The nucleus becomes condensed and is surrounded 

 by a very thin layer of cytoplasm in the head of the mature spermatozoon. 

 The centrioles and a small mass of cytoplasm form the middle piece, and the 

 third part is the tail surrounding the flagellum, by means of which the mature 

 male germ cell, or microgamete, is able to swim (Fig. 5.3/1). The spermato- 



Cytoplasmic 



/ 



Head 



Flagellum 

 Cytosome 

 Nucleus 



Mid piece 



Acrosome 



Tail 



Fig. 5.8. Stages in spermiogenesis in the guinea pig. A, spermatid with early growth of 

 flagellum from one centriole. B and C, showing the centrioles near the nucleus, the accumula- 

 tion of cytoplasm along the flagellum, and the formation of the acrosome, which is the most 

 anterior part of the spermatozoon. D, mature sperm, showing head, midpiece, and tail; the 

 cytoplasmic mass has been detached and lost. (From F. Meves, 1898, Arckiv fiir mikroscopische 

 Analomie, vol. 54.) 



137 



