DISTRIBUTION >>F REPRODUCTIVE METHODS 267 



purposes are single cells which are capable of developing into 

 new individuals, and hence the term reproductive cells better 

 describes them. Of these reproductive cells we recognize the 

 following kinds : 



Spores: single-celled reproductive bodies, capable of growing 

 into new organisms without uniting with a sperm. 



Eggs, or ova: large, stationary cells, which grow into new 

 individuals only after uniting with a sperm. 



Sperms: minute, usually motile cells, which must unite with 

 an egg to enable it to develop. 



Parthenogenetic eggs: large, stationary cells, resembling, or 

 identical with, eggs, but able to develop without union with 

 a sperm. 



The name gametes (Gr. gamete = wife or husband) is fre- 

 quently applied to the cells that unite with each other in cell 

 union. This term, therefore, includes eggs and sperms, and also 

 the uniting cells in conjugation where no distinction of sex 

 is seen. 



CROSS FERTILIZATION THE RULE 



Cross Fertilization. In ordinary sexual reproduction the 

 rule is that a single sperm unites with a single egg. When 

 the sexes are separate, as in the frog, this will always result 

 in the fertilization of an egg from one individual with a sperm 

 from another. As we have seen, some animals produce both 

 eggs and sperms, and might fertilize their own eggs. But 

 usually there is some device to prevent this. In the earth- 

 worm, although both eggs and sperms are produced by each 

 individual, in copulation there is an interchange of sperm 

 fluid, in such a way that the eggs of each individual are sub- 

 sequently fertilized by the sperms from the other. This is 

 called cross fertilization. In most cases where both male and 

 female organs are produced in the same individual, there is 

 some device by which cross fertilization is insured. In the 



