the student at this point. How do we know that the 

 structure that bears the spores in the moss is really a new 

 generation and not merely a part of the regular moss 

 plant, as it looks to be? That it is a new generation is 

 shown by the fact that it is produced by the development 

 of a fertilized egg. We know always that a new generation 

 begins when we start with a fertilized egg. 



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 diflfer 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 

 many attached individuals that have arisen by budding 

 from a single parent. After a period of this kind of 

 budding, the colony forms other buds of a different shape 

 and structure. When these mature they detach themselves 

 and become free swimming jelly-fish. No naturalist, 

 discovering them swimming in the water, unless he knew 

 the facts just stated, would ever suppose for a moment 

 that these jelly-fish were in any way related to the colony 

 from which they actually came. The free jelly-fish 

 produces gametes, which unite sexually and form embryos. 

 When the embryos develop they are not like their parents 

 the jelly-fish, but are like their grandparents, the tubular 

 hydroids. They settle down and reproduce by budding. 



Many such instances are found in the coelenterates (that 

 branch of the animal kingdom to which the hydra belongs) 

 and among the parasitic wormSj such as liver ilukes, tape 

 worms, and the like. 



6. Some Advantages in Alternation of Generations in 

 Animals. We do not know what forces have brought 

 about alternation of generations, but we can see that it 

 may be of some advantage to the organisms having it. It 

 gives a kind of double chance for organisms to scatter and 

 to become adapted to life conditions. For example, in 

 attached forms such as hydroids, the budding enables a 

 single individual that manages to become attached in a 

 favorable place gradually to take complete possession of 

 that spot by budding. Being attached has advantages, but 



it also has handicaps. If conditions change, the colony 

 may be destroyed. Besides, there is no chance in budding 

 for wide distribution. Now the formation of another kind 

 of individual which is free-swimming, and able to produce 

 eggs and sperms, gives the fixed colony a chance to spread 

 its offspring widely and to seek out many favorable 

 localities for growth. In this way the advantages of both 

 methods are combined in one species. 



In the parasitic form, the alternation is often an aid in 

 getting back and forth from one species of host to another, 

 as is so often necessary. For example, the liver fluke is 

 in the snail for a while, in the liver of the sheep a while, 

 and in the water of the pond between times. It is during 

 these alternations of generations, with their differences of 

 structure and habits and instincts, that these perilous 

 changes are made. 



CHAPTER FIFTEEN. 

 SOME EGGS THAT ARE NOT FERTILIZED. 



1. The Fate of Eggs. As we have seen, some of the 

 eggs formed by plants and animals are fertilized by the 

 sperms and then may develop into mature plants or 

 animals. But a great many fail to be fertilized, and such 

 eggs, no matter how perfect they are, cannot preserve 

 their life under ordinary circumstances. Such finally die 

 and decompose. Investigators have found that they can 

 take the eggs of many of the lower animals while they 

 are in good condition, even though they have not been 

 fertilized, and, by changing the external conditions about 

 them in certain ways, stimulate them to begin development 

 as though they had been fertilized. (See page 00). 



2. Parthenogenesis. There is one other exception which 

 is so remarkable as to demand attention. Quite a number 

 of organisms regularly produce eggs that develop into 

 the adult without fertilization by the male cell. We do 

 not know whether they are stimulated in some special 

 way while in the body of the parents, or have the power 

 within themselves to develop without any stimulus. This 

 development without fertilization is parthenogenesis. The 

 circumstances under which it takes place are different in 

 different organisms. In some cases, as in the queen bee, 

 the mother can, apparently, determine at any time 

 whether the eggs shall be fertilized or not. In other cases, 

 as in some of the rotifers (microscopic animals), the non- 

 fertilized eggs ("summer eggs") are laid at certain seasons 

 of the year and the eggs requiring fertilization ("winter 

 eggs") are produced at other seasons. In some cases the 

 mothers may differ, some forming both summer and winter 

 eggs and others parthenogenetic eggs only. 



Furthermore, there is occasional parthenogenesis even 

 among forms where fertilization is the rule. For example, 

 it is known that the s^porophyte of ferns may sometimes 

 develop from cells of the gametophyte without fertilization. 



3. The Case of the Honey Bee. Perhaps the best known 

 case of parthenogenesis is that of our common bees. The 

 queen, who is the one perfect female of the hive, receives 

 a large supply of sperms at mating. This is stored in a 

 sac opening into the duct along which eggs come from the 

 ovaries on their way outwards. It is believed that the 

 queen can control the sperm supply in such a way that 

 eggs may pass to the surface either with sperm or without. 



