THE CHROMOSOMES AND HEREDITY 387 



But when the reduction division occurs the paternal and 



maternal elements are separated and the result is 



either or 



(P.M.) 

 2dp.b.= I (P.) (M.) J 



(P.M.) 

 (P.)(M.)J 



2d p. b. 



= ist p. b.= 

 = mature eggs = 

 If the first polar body divides the result is 



\'i ) ^i \ in which either one of 

 (P.) (M.)J 



the four cells may represent the ripe egg and the other 



three the three polar bodies. The ova are, therefore, either 



(P)aternal or (M)aternal in regard to the hybrid character, and 



the two kinds are probably equally numerous. 



786. In the maturation of the sperm the process is similar. 



The primary spermatocyte (P^M^) divides into two secondary 



spermatocytes, 



(P' MM 



/^/' ^./N which then divide into 



(P'. M'.) 



(P'.) (M^) , ., ^ . -, /^A . 



/p, X (^r \ four spermatids. Two of these are (Pjater- 



nal and two are (MOaternal, so that the number of (P^) and 

 (M'.) sperm is also equal, as in the case of the egg. 



787. By close fertilizing, the gametes of the Fi hybrid 

 generation may combine as follows: 



(P.P'.), (P.M^), (P'.M.), (M^M.). This results in one-fourth 

 pure paternal, two-fourths hybrid and one-fourth pure maternal 

 individuals in the F2 generation. 



788. Number of Species. — In spite of the fact that it is 

 often extremely difficult and even impossible to definitely cir- 

 cumscribe a species, yet for convenience of reference animals 

 and plants must be divided into convenient groups, named and 

 classified. And much time has been spent in hunting out and 

 describing species. About 520,000 animal species have thus 

 been listed and over 200,000 plants. Besides these there are 

 many species which have become extinct and are now only 



