172 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



fusion 



they conjugate. By a 

 series of divisions, such 

 as is shown in the dia- 

 gram, the number of 

 chromosomes in the nu- 

 clei of the zygote, result- 

 ing from the union of the 

 two gametes, is reduced 

 . . to half this number. If 

 \ spelling disappear g^j^^g g^^j^ dcvice as this 



' ^ / \ were not used, every time 



sex cells united, the num- 

 ber of chromosomes 

 would be doubled. How- 

 ever, by this so-called 

 reduction division during 

 the formation of the ga- 

 Diagram to show how reduction division takes metes, which OCCUrs in 

 place in the zygote of Spirogyra. both plants and animals, 



the number of chromosomes is halved. We speak of the single number 

 of chromosomes as haploid and the double number, which comes with 

 the union of the two gametes, as diploid. 



■2.y^U 



first division, 

 reduction 

 of ci-ji-TomoSomcS 

 from ?T-; to ri , 

 mat.u.ra.t,ion 



/second 

 cLivision, 

 TTJitjDSiS 



Oedogoniiim 



In another of the filamentous algae, 

 Oedogonium, there is the first appearance 

 of two kinds of sex cells. This alga repro- 

 duces by zoospores and in addition forms 

 two sex organs, structures called anther- 

 idia, which produce a number of ciliated 

 sper7n cells and oogonia, the latter holding 

 a single egg cell. The sperm cells swim 

 through the water from the antheridia, 

 one uniting with the egg cell, and almost 

 immediately a thick wall is formed about 

 the fertilized egg. This oospore does not 

 produce a new plant directly, but gives 

 rise to zoospores, which in turn eventu- 

 ally become new plants. 



Life history of Oedogonium. 



