ALTERNATION OF GENERATIONS 



modification, it is an essential part of the life cycle of nearly all 

 species. 



Sexual reproduction consists superficially in the formation and 

 separation from the rest of the organism of single cells, the germ- 

 cells or gametes (constituted by the whole organism, in the Protista, 

 and by a specialised part of it, in the higher plants and animals) and 

 their subsequent fusion in pairs to give new cells, known as zygotes. 



Its essential genetic characteristics are two. The first was seen 

 by Oscar Hertwig (in 1875) to consist in the fusion of the nuclei 

 of the two gametes. Since they carry the same number of 

 chromosomes the product has a double or diploid number. This 



TUl 



US 



Tkallu, 



Cells 



TV- 



Fig. I . — The life cycle of an alga such as (Edogoniiim . A haploid thallus 

 produces haploid sperm and egg cells («) which fuse to give a 

 diploid zygote (2«). This undergoes immediate meiosis to give 

 haploid spores from which a new thallus develops. 



h fertilisation. The second was predicted by Weismann (in 1887). 

 It consists in the compensating process of reduction or meiosis. 

 In a mother-cell two nuclear divisions follow one another rapidly 

 while the chromosomes only divide once, so that four nuclei are 

 formed, to each of which a halved or haploid number of chromosomes 

 is distributed. 



The recurrence of fertilisation and meiosis gives two types of 

 nuclei in each sexually reproducing species, the diploid or 

 double nucleus of the zygote and the haploid or halved nucleus 

 of the gamete. Either or both of these may, in different species, 

 continue to multiply by mitotic divisions, giving a number of 

 unicellular organisms or one multicellular one, so that we have 



