LIFE HISTORY OF A FERN 173 



with the sperm, possesses a double or diploid nature. 1 

 In recognition of its dual nature it is called the oosperm 

 (egg and sperm). 2 The oosperm, however, like the un- 

 fertilized egg, is still only one cell, though its nucleus com- 

 prises substances contributed by both egg and sperm. 

 In some cases the egg and sperm that unite in fertilization 

 may come from different parents; their fusion is then 

 called cross-fertilization. 



FIG. 131. Young embryo of a maidenhair fern (Adiantum concinnum); 

 still surrounded by the archegonium, which has grown in size. L, leaf, 

 S, stem; R, root; F, foot. (After Atkinson.) 



158. Development of the Fertilized Egg. After fertili- 

 zation the egg begins to develop, undergoing a series of 

 nuclear and cell-divisions, accompanied by increase in 



1 As distinguished from the unfertilized egg, which is of a single, or 

 haploid nature. 



2 The term oospore is often used here, but this term lacks the advan- 

 tage of indicating the real nature of the fertilized egg. 



