ONTOGENY OF METAZOA 151 



indefinitely, but must give place occasional!}' to sexual 

 processes, and the great majority of animals cannot re- 

 produce parthenogenetically at all. 



In spite of the various ways in which individuals may 

 arise, the development of any particular metazoan from a 

 fertilized ovum usually shows a rather definite sequence of 

 events, and this series of stages is practically the same in 

 all the different phyla, except one. Before passing on to 

 the phyla of metazoans, it will be proper therefore to 

 consider a typical developmental history. 



ONTOGENY OF A TYPICAL METAZOAN 



1. Maturation (Fig. 64, A, B). A many-celled animal 

 starts as a single cell, the zygote, formed by the union of the 

 egg cell, or ovum, with a sperm cell, or spermatozoon. Be- 

 fore a zygote can be formed, however, there must be cer- 

 tain preparatory or ripening changes in both the egg and 

 sperm, and such changes are known as maturation. The 

 egg cell during its maturation undergoes mitotic cell divi- 

 sion twice; but at each division (&, c), instead of two cells of 

 equal size being formed, one cell is very large and the other 

 minute. The smaller cell is known as a " polar body," 

 and is to be looked upon as a rudimentary egg which has 

 been sacrificed in order to leave the functional egg as much 

 nourishment as possible. The maturation of the sperm 

 cell (B) consists also of two successive cell divisions (b, c), 

 but in this case four functional spermatozoa of equal size 

 are formed (d), each bearing a little tail for swimming. 



But, you ask, why not have the egg and sperm fuse at 

 once without ripening why have maturation at all? 

 There is apparently but one important thing involved. 

 During the two cell divisions chromatin material is elimi- 

 nated and the number of chromosomes is reduced to one- 

 half the somatic number. If the somatic cells in a par- 

 ticular animal showed ten chromosomes during mitosis, 

 a ripe germ cell would have only five. 



