INBREEDING AND CROSSBREEDING 237 



egg-cell may result from its fertilization by the foreign sperm. 

 Thus when the egg of a sea urchin is fertilized with the sperm 

 of a sea lily, an animal of a wholly different class of echino- 

 derms, the egg begins development following a fusion of the 

 sperm and egg nuclei, but the nuclear substance introduced 

 by the sperm soon degenerates and disappears. The egg, 

 however, having once started to develop, continues to do so, 

 producing an organism showing only characters of the ma- 

 ternal species. Its development is as truly parthenogenetic 

 as when induced by chemical or osmotic means, as is now 

 known to be possible in the case of the eggs of many marine 

 and of some fresh-water animals. Thus the unfertilized egg 

 of a frog may be made to develop by chemical means (or 

 even by puncturing the superficial layer of the egg with a 

 needle), a process we may call artificial or induced partheno- 

 genesis. Now in crosses of species too widely separated to 

 produce a hybrid individual, the sperm may merely induce 

 parthenogenesis. This method of inducing parthenogenesis 

 is being used by plant breeders of the United States Depart- 

 ment of Agriculture to obtain orange seedlings which it is 

 hoped may be superior to the mother plant in certain re- 

 spects, though the progeny will inherit none of the qualities 

 of the pollen plant. It is hoped merely that there may occur 

 in the parthenogenetic offspring some segregations or vari- 

 ations of the characters found in the mother plant. 



What might be called male parthenogenesis has been re- 

 ported in crosses of strawberries made many years ago by 

 Millardet and also in a cross between Mexican teosinte, a 

 plant related to maize, and a coarse grass of the southern 

 United States. (Collins.) In such cases a cross-fertilized seed 

 produces a plant which shows only characters of the pollen 

 parent. It is supposed that the egg nucleus has taken no 

 part in the production of an embryo, but that this has arisen 

 wholly from nuclear material of the pollen tube. 



Considering all the facts, changes in heterozygosity alone 

 seem an insufficient explanation of the effects of crossing and 

 inbreeding respectively. It is necessary to suppose further 



