202 THE PATH OF DEGENERATIVE EVOLUTION 



We may give a brief account of the formation of 

 the sexual cells in Selaginella. The first stages in 

 the appearance of microsporangia and macro- 

 sporangia are identical. Later on in the one case 

 each of the spore mother-cells breaks up into four 

 microspores, in the other case only one mother-cell 

 breaks up into four macrospores, the others dis- 

 appear. On germination the macrospore becomes a 

 fairly large prothallus with several archegonia : the 

 microspore forms only one cell as the representative 

 of the prothallus, the rest of the structure becoming 

 the antheridium. 



In Phanerogams (flowering plants) a drop of 

 water is no longer necessary for fertilization. The 

 spermatozoon reaches the ovum, not by swimming 

 movements, but by a kind of growth. This 

 difference has resulted in profound modifications. 

 Fertilization may occur in the air, and it is no 

 longer necessary for the macrospore to become 

 detached : it remains fixed on the individual from 

 which it arose and there proceeds to develop. 

 In the Gyrnnosperms the archegonia are fully 

 formed in a prothallus of considerable size, but the 

 Angiosperms which are more specialized, retain 

 only traces of these structures. The microspores 

 (pollen grains) exactly as in Selaginella are pro- 

 duced in great numbers, but when they leave the 

 parent plant they develop only if they reach suit- 

 able conditions. In germination there is shut off, 

 as in Selaginella, a single cell to represent the 



