ALTERNATION OF GENERATIONS 81 



In mosses and ferns, then, we have a good example of 

 alternating generations, in which the spore is the beginning 

 of the gametophyte generation of the plant, and the 

 fertilized egg is the beginning of the sporophyte 

 generation. 



4, The Behavior of the Chromosomes in the Life Cycle 

 of Mosses and Ferns. In Chapter Eleven it was shown that 

 mature eggs and sperms have only X chromosomes in 

 their nuclei before they unite, 2 X being the number found 

 regularly in the body cells. When the gametes have 

 united, the number of chromosomes is thereby doubled. 

 The alternation of generations in the moss and fern shows 

 this in a very interesting way. If we start with the 

 fertilized egg, after the union of the gametes has brought 

 the chromosomes up to 2 X, we find that the following cell 

 divisions give to all the daughter cells of the developing 

 sporophyte 2 X chromosomes. This continues to be true 

 until just before the spores are to be formed. At this 

 time a reduction division occurs, similar to that described 

 in Chapter Ten, and the spore has only X chromosomes. 

 When the spore germinates, and divides to form the 

 gametophyte, its descendant cells continue to have X 

 chromosomes all through the life of the gametophyte. 

 The gametes formed in this gametophyte also possess the 

 X number of chromosomes, and are consequently ready to 

 unite. At their union the nucleus becomes 2 X once more. 



5. Alternation of Generations in Animals. Alternation 

 of generations is much more common in plants than in 

 animals, but we have some very interesting examples 

 of it among animals. Indeed, we first knew of it among 

 animals. In the group of animals to which the 

 fresh water hydra belongs, there are many species similar 

 to hydra. Most of them differ from hydra in that 

 when they form new individuals by budding, the new 

 individuals do not escape and become independent, but 

 remain attached. The result of this is finally a colony of 



