210 THE PERPETUATION OF ADAPTATIONS 



Reduction in number of chromosomes also occurs in the plant 

 world. Here, in many cases, the germ cells with a reduced 

 number continue to proliferate, and even to give rise to an 

 entire plant (gametophy te) . Thus in the fern, reduction in the 

 number of chromosomes occurs at the time of formation of the 

 spores cf. p.i2i. Each spore, and all of the cells of the sexual 

 generation formed from it (pro thallium) , thus have only one- 

 half the number of chromosomes contained in the cells of the 

 asexual generation (sporophyte), the full number being 

 restored by union of the spermatozoid and the oosphere. 

 While details differ slightly in animals and plants the essen- 

 tial facts are the same. 



The male cells resemble the female in the germinal tissue, but 

 in many types of animals characteristic changes soon appear 

 which make these early germ cells distinctly different from 

 early egg cells. These differences have to do with the deter- 

 mination of sex which will be considered in a later section. In 

 other types of animals no such visible sex-indicating differences 

 appear and in these the nuclear changes, formation of tetrads 

 in haploid number, and double division take place as in the egg. 

 No polar bodies are formed, but four functional spermatozoa 

 result (Fig. 90). 



The Germ Cells after Maturation. If, on Weismann's hypo- 

 thesis, the chromosomes are made up of a series of factors 

 determining adult structures they must be unequally dis- 

 tributed in the germ cells. In Ascaris, a nematode worm, 

 for example, there are four chromosomes in the early germ 

 cells. We may follow a hypothetical group of characters in 

 one of these in spermatogenesis as shown in Fig. 91. The 

 light end on one of these four chromosomes (A and B) 

 represents such a group. In the first maturation division 



(C) this group passes undivided into one of the daughter cells 



(D) while the other daughter cell (D') contains no part of it. 

 At the second maturation division (D) the group is equally 

 divided by a longitudinal division of the chromosome, while D' 

 divides into two cells with no part of the group. Of the four 

 spermatozoa which result, two (E, E) contain the group of 



