48 Reproduction and Meiosis 



cells or gametes. Some lower forms reproduce sexually by the 

 fusion of identical gametes, but a differentiation of the gametes 

 into male and female is the rule in the higher groups. 



Reproduction and Life Cycles in Higher Animals 



Sexual reproduction in the higher animals is generally brought 

 about by the union of two unlike gametes, each of which is 

 contributed by a different individual. One gamete, the egg, is 

 large and nonmotile, is produced by the female, is usually 

 spherical, and contains a nucleus and cytoplasm. In the cyto- 

 plasm is found the food or yolk. The male gamete, which is 

 much smaller than the egg, is known as the spermatozoon or 

 simply the sperm. It usually consists of three parts: the head, 

 middle piece, and tail. The head is essentially a nucleus sur- 

 rounded by a very thin layer of cytoplasm and is generally 

 spherical or elliptical in shape. The middle piece is much 

 smaller than the head and, at least in some animals, contains 

 a centrosome. The tail is very long, extremely delicate, and is 

 a flagellum which propels the sperm from place to place. The 

 union of an egg and a sperm is called fertilization. This process 

 results in a cell, the zygote, which will develop into a new adult 

 individual of the same species as the parents. 



It was stated previously that every cell of the body, with the 

 exception of the germ cells, contained the same number of 

 chromosomes. In man, the number in the somatic cells is 48 

 chromosomes or 24 pairs. If the germ cells were produced by 

 typical mitotic divisions and, therefore, if they had the same 

 number of chromosomes as the body cells, the number of 

 chromosomes of a species would double each generation. Thus 

 it would not be long before the chromosome number of all or- 

 ganisms would be in the thousands and even millions. Actually, 

 however, this does not happen for, with the development of sexual 

 reproduction, a modification of the ordinary mitotic process has 

 developed which, in animals, produces gametes with half the 

 number of chromosomes as in the body cells of the parents. Thus 

 in each generation of human beings the body cells have 48 chro- 

 mosomes and the gametes have 24. In discussing animals in 

 general, without reference to any particular species, it can be 

 said that the body cells contain 2n chromosomes and the germ 

 cells contain n, where n stands for a definite number. This num- 



