26o SPERMATOGENESIS AND OOGENESIS LESS. 



The formation of both polar cells takes place by a 

 reducing division, so that, while the immature ovum con- 

 tains double the number of chromosomes found in the 

 ordinary cells of the species, the mature ovum, like the 

 sperm, contains only one-half the normal number. 



In some animals the first polar body has been found to 

 divide after separating from the egg. In such cases the egg- 

 mother-cell or immature ovum gives rise to a group of four 

 cells the mature ovum and three polar-cells ; just as the 

 sperm-mother-cell gives rise to a group of four cells, all of 

 which, however, become sperms. 



After maturation has taken place, the ovum is ready to be 

 fertilized by the conjugation with it of a single sperm. As 

 we have found repeatedly, sperms are produced in vastly 

 greater numbers than ova, and it often happens that a single 

 egg is seen quite surrounded with sperms, all apparently 

 about to conjugate with it. It has however been found to 

 be a general rule that only one of these actually conjugates : 

 the others, like most drones in a hive, perish without fulfill- 

 ing the one function they are fitted to perform. 



The successful sperm (B) takes up a position at right 

 angles to the surface of the egg, and gradually passes 

 through the micropyle (tnicrop) or works its way through 

 the vitelline membrane until its head lies within the egg- 

 protoplasm. The tail is then cast off and the head, ac- 

 companied by the intermediate piece or centrosome, pene- 

 trating deeper into the protoplasm, takes on the form of a 

 rounded nucleus-like body, the male pronudeus (c, $ proji). 



The two pronuclei approach one another (D) and finally 

 unite to form what is called the segmentation nucleus (E, seg. 

 v,ucl\ the single nucleus of what is not now the ovum but 

 the oosperm the impregnated egg or unicellular embryo. 

 The fertilizing process is thus seen to consist of the union 



