stant through the sexual or gametophyte stage, which is 

 also called the x generation. When fertilization occurs 

 each gamete contributes its quota of chromosomes, thus 

 producing the 2x generation. 



In the case of ferns, the spores germinate and give rise 

 to prothallia; on the prothallia develop the gametes — male 

 and female— still with the reduced number of chromosomes. 



By fertilization or fusion of the gametes a zygote is pro- 

 duced with the normal number of chromosomes. In flower- 

 ing plants the gametophyte stage is much further re- 

 duced. 



The formation of the sex cells, sperms or eggs, in the 

 higher animals may be represented by the following dia- 

 grams: 



FIG. 24. 

 Ogenesis. Polar Bodie.s and Reduction; A — Grandmother 

 egg-cell of Ascaris with four chromosomes; B — Fully devel- 

 oped egg mother-cell with eight chromosomes, the result 

 first of fusion in pairs to form two bivalent chromosomes, 

 then the formation of tetrads: C — Oocyte with four chromo- 

 somes, and formation of first polar body (I'bi) with four 

 chromosomes, which soon divides into two polar bodies with 

 two chromosomes each; D — Formation of second polar body 

 (Pb2). The nucleus of the new mature ovum has two ohro- 

 mosomes. (After Hertwig). 



FIG. 25. 

 Spermatogenesis and Reduction: A — Grandmother sperm- 

 cell of Ascaris with four chromosomes: Bi, sperm mother-cell 

 with eight chromosome.s, the result first of fusion in pairs to 

 form two bivalent chromosomes, then the formation of tet- 

 rads; Ci, two spermatocytes, each with four chromosomes: 

 Di. four spermatozoa formed each with two chromosomes. 

 (After Hertwig). 



134 



