52 REPRODUCTION 



animal now has part of its own nucleus and part of that 

 of another animal. After this the animals separate. This 

 differs in several respects from the other conjugations 

 described above. The union of the animals is temporary. 

 Each, however, contributes a portion of its nucleus 

 permanently to the other. The number of individuals is 

 not diminished as in other cases we have described. There 

 has just been a change in the inner substance of each 

 animal. After a union of this sort the Paramecium has a 

 period of reproduction by fission. 



6. Must These Unions Occur? In some cases it appears 

 clear that the unions, while common, are not absolutely 

 necessary. Often, even in forms that normally unite, the 

 gametes may develop at once without union. Taking the 

 animal and plant kingdom as a whole, however, the 

 gametes do not find it easy to develop freely into the adult 

 organism without conjugating. In some types the union 

 seems to be absolutely necessary. 



7. What is the Value of the Union? What possible 

 reason or value these unions have is one of the most 

 interesting questions in biology. We know that they must 

 have some meaning in life because they are so nearly 

 universal. We are not perfectly sure just what the 

 advantage is, but in some way it is prophetic; it looks to 

 the quality of the future generations. It is pretty well 

 established that the stock formed by uniting two cells 

 from different strains is better, generally speaking, than 

 that formed from a single strain. It seems to have more 

 continuous vigor and more variety of development. It 

 appears that the plant or animal, which results from the 

 union of two cells, is less liable to be just like either 

 parent than one resulting from a single parent only. The 

 uniting cells seem to stimulate one another, and, perhaps, 

 to produce a kind of renewal of youth. 



