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, 
5, stem; R, root; F, foot. (After Atkinson.) 
158. Development of the Fertilized Egg. Af ter 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. 
