272 CONCLUDING CHAPTER 



place in sets of four, and their production is preceded by 

 a reducing division, so that each spore nucleus contains 

 half as many chromosomes as the nuclei of the fern- 

 plant the spores, in fact, represent the initiation of 

 the x generation. 



Spores take no part in any process of conjugation. 

 They at once germinate and enter on an embryonic 

 development of their own, giving rise to a considerable 

 mass of cells, all of which contain the reduced number 

 of chromosomes. Thus in the case of the fern we have 

 a small but well-developed %-generation alternating 

 with a much larger 2#-generation. The mass of 

 cellular tissue making up the ^-generation has been 

 named the prothallus. 



Certain cells of the prothallus develop, without 

 change in the number of their chromosomes, into the 

 gametes. These are differentiated in the usual way 

 into male and female ova and spermatozoids respec- 

 tively. 



Fertilization of the ovum by the spermatozoid gives 

 rise to a zygote in which the double number of chromo- 

 somes is restored. In this way the 2%-generation or 

 fern plant is initiated, and by the usual processes of 

 cell multiplication and differentiation this body 

 becomes completed, developing its characteristic fronds 

 and so forth. Thus in the ferns the 2#-generation has 

 arrived at a high degree of development, and represents 

 the chief bulk of the plant. The ^-generation, however, 

 still embodies a considerable mass of cells. 



Turning to the higher plants, among which we may 

 include those which produce typical flowers with 



