30 ZOOLOGY. 



active, and with thin protoplasmic projections (Fig. 9). 

 Structurally, the typical spermatozoon consists of a " head " 

 piece, a middle piece, and a " tail " or flagellum. The head 

 is composed chiefly of the chromatic material of the nucleus. 

 A delicate covering of cytoplasm envelopes the head and is 

 drawn out into the projection known as the tail (Fig. 9, D). 

 The middle piece has been variously interpreted. Some re- 

 gard it as the achromatic part of the nucleus, others as con- 

 taining the centrosome of the male cell. 



48. Maturation of the Ovum. After the egg cell has been 

 produced by the maternal germinative tissue, and before the 

 union of the male and female cells, the nucleus of the egg- 

 cell approaches the surface of the cell and divides twice in 

 close succession by the indirect or mitotic method (see Fig. 

 10). The cytoplasm of the ovum does not divide equally. 

 A small amount of cytoplasm enclosing a half of the nuclear 

 material forms a bud-like cell on the surface of the ovum dur- 

 ing each of the two acts of nuclear division. These minute 

 cells, the polar bodies, are cast off from the egg and perish. 

 They are to be regarded as abortive or vestigial eggs. In the 

 egg-nucleus there remains therefore only one-fourth of the 

 original nuclear material. The remnant returns to the cen- 

 ter of the cell and is termed the female pronucleits. It con- 

 tains only one-half the number of chromosomes found origin- 

 ally in the ovum. This (elimination of the nuclear material 

 including the reduction of the chromosomes is known as the 

 maturation or ripening of the ovum) The abortive cells or 

 polar bodies are not known to have any function other than 

 this elimination of material. The nuclei of the sperm cells 

 during their formation undergo a similar reduction of the 

 chromosomes but in a somewhat different way. In these are 

 no abortive cells. All the daughter cells form sperm. 



49. Fertilization. The union of a sperm cell with the 

 ovum constitutes the act of fertilization. Often there is a 

 special aperture (micropyle) in the outer egg-membrane 



