GERM CELLS 215 



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 (gametophyte). Thus in the fern, reduction in the 

 number of chromosomes occurs at the time of formation of the 

 spores (cf. p. 122). Each spore, and all of the cells of the sexual 

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

 half the number of chromosomes contained in the cells of the 

 asexual generation (sporophyte), the full number being re- 

 stored by union of the spermatozoid and the oosphere. While 

 details differ slightly in animals and plants, the essential 

 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 differ- 

 ences 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 variously distributed 

 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 sper- 

 matogenesis, as shown in Fig. 91 (upper series). 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 matura- 

 tion 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 characters which is not 



