PRODUCTION OF SPERMS 



285 



lively large size, due to the yolk in them, and the fact that they 

 are incapable of independent motion. 



Production of sperms. The pair of organs that secretes the 

 male gametes or sperms is the testes. The sperms, too, in the 

 maturation process lose half their chromosomes. By means of 

 tubes the mature sperms of the frog pass from the testes into the 

 cloaca and out of the body. These sperms are known as milt. 

 Each sperm is a single cell consisting largely of a nucleus. The 

 cytoplasm is drawn out to form a tail. By means of the tail 

 lashing back and forward, the sperm swims in the water. (Prob- 

 ably a chemical attraction draws the sperm to the egg.) The 

 nuclei of the sperm and egg fuse in the fertilization process, form- 

 ing a fertilized egg cell, the one-celled embryo. In this process 

 the original number of chromosomes is restored. 



The entrance of the sperm into the egg effects the initial devel- 

 opment of the egg as well as brings about variation by combining 

 the chromosomes from two parents. Each fertilized egg contains 

 one-half maternal and one-half paternal chromosomes. 



Development of the frog. The fertilized egg or embryo absorbs 

 food from the yolk. It grows and divides mitotically to form a 

 two-celled stage. These 

 two cells feed, grow, 

 and divide to form a 

 four-celled stage. Mi- 

 totic divisions con- 

 tinue, forming various 

 many-celled stages 

 called cleavage stages. 

 Finally, the solid mass 



or cells, tne morula, primary sex ceUs are formed early in the life of an organism, 



becomes arranged in They contain the same number of chromosomes, 2x, as body 



° ^ cells. In the maturing of the sex cells to form sperms and 



the form of a single- eggs, the chromosomes become reduced to half the number, 



J^ X. When the sperm unites with the egg to form the zygote, 



layered hollow ball of the 2x number is again restored. 



