ONTOGENESIS 183 



plant. In such cases there is no reduction of the 

 number of chromosomes. There is evidence that 

 the initiatory " stimulus " of development may be 

 derived from contact of the egg with the surrounding 

 endosperm. In the case of the fig (Ficus), the 

 puncture made by a tiny wasp 1 (Blastophaga) is 

 probably the cause of development. 



Apogamy. Very similar to true parthenogenesis, 

 and much more common, is the production of a 

 sporophyte by a gametophyte from other sources 

 than the egg-cell, but still without fertilization. 

 This is termed apogamy. Sometimes the embryo 

 is produced by growth of gametophyte tissue (fre- 

 quently in ferns), and this is to be classed as simple 

 budding. In other cases, the embryo develops by 

 the cleavage of the synergids, which are to be con- 

 sidered as abortive eggs. This is common in the 

 orange, the dandelion, and many conifers. In a 

 very similar way, among the ferns, prothallia 

 (gametophytes) may be developed directly from the 

 leaflike sporophyte, without the intervention of a 

 spore. This is called apospory. 



The Probable Evolution of the Plant World. The 

 simplest forms, both in animal and plant life, are 

 aquatic, and life appears to have begun in the water. 

 In an aqueous medium, free-swimming organisms 

 can go in any direction, and the conjugation of 

 gametes is effected with relative ease and certainty. 



1 It is of interest to note that in the frog's egg, development may be 

 induced by puncturing the unfertilized egg with a needle. 



