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. 



CHAPTER X. 

 EGGS AND SPERM. 



1. Conjugation the Starting Point. There have been 

 no differences in the gametes or mating bodies in those 

 species we have studied. The ciliated gametes of Ulothrix 

 are of the same size and structure. The mating cells of 

 Spirogyra are apparently alike. The paramecia that unite 

 are as nearly alike as it is possible for organisms to be. 

 Indeed, we call the process conjugation only when the 



• gametes are alike, so far as we can perceive. This 

 condition must be looked upon as the simplest kind of 

 union of offspring. It seems to be the beginning of a 

 process which is very common in both plants and animals. 

 This we must now study. 



2. Beginning of Differences in Gametes. Pandorina 

 is a simple green plant made up, when mature, of sixteen 

 cells held together by a jelly which they have secreted. 

 It is not a plant you are likely to see, but it is 

 described in both botanies and zoologies, because it is 

 hard to say whether it is more like plants or animals. It 

 starts as a single cell. By a series of divisions the mature 

 colony of sixteen cells is finally formed. Any one of these 

 sixteen cells may start a new colony by dividing again. 

 Or, instead of this, any one or all of these' cells may 

 divide into a number of gametes, which are small and 

 ciliated. These escape from the jelly into the water and 

 unite in pairs. The gametes are almost alike, but differ 

 more or less in size. A smaller and a larger unite. 



In Eudorina, which is very much like Pandorina, there 

 is much more difference in the size of the two kinds of 

 gametes. Some of the gametes are quite large and plump. 

 Others are small and spindle-shaped. The differences are 

 at once recognized under the microscope. One of the thin 

 cells unites with one of the plump ones. A small one 

 never unites with a small one, or a large one with another 

 large one. The result is the same as in conjugation. But 



we call the process fertilization when one gamete is much 

 larger than the other. The small gamete is said to 

 fertilize the large one. This name expresses an early 

 guess as to the value which this union has. It suggests 

 that the small cell stimulates or nourishes the large one 

 for a better development. But we now have good reason 

 to think that the relation is a somewhat deeper one. Each 

 gamete makes a definite contribution to thie individual 

 which results from the combination. 



3. Ovum and Sperm. What has just been described is 

 the beginning of the differences which we find in the 

 offspring (gametes) of all the higher plants and animals. 

 The larger of the gametes is an egg, the smaller is a 

 sperm, and it may be very small indeed as compared with 

 the egg. The difficulty now is not to tell them apart, but 

 to find anything in which they are alike. In appearance 

 they differ about as much as cells can. Both, however, 

 are single cells. In the egg there is a large amount of 

 nourishment stored by the parent. On the other hand, 

 the sperm is a cell with little more to it than a nucleus. 

 It carries no nourishment, but is usually exceedingly 

 active. AH the higher plants and animals produce these 

 two kinds of cells. 



4. Eggs and Their Characteristics. An egg, as we have 

 seen, is a cell. It may have some very peculiar structures 

 and powers, but it must always be remembered as a cell. 

 The shell and the white of a hen's egg are not parts of the 

 egg in this sense. Scientifically speaking, the yolk is the 

 egg proper, while the shell and the white are merely 

 protecting and nourishing structure about it. The yolk 

 is a single very large cell. 



Eggs usually have large nuclei, and a good supply of 

 protoplasm. They are without power of motion. They 

 vary in size from tliat of the ostrich down, and until they 

 are too small to be seen with the naked eye. There is 

 little connection between the size of the animal or plant 

 and the size of the egg it produces. The eggs of humming 

 birds are many hundred times as large as those of trees 

 or of human beings. Eggs are plump, well-nourished, 

 and sluggish cells. They need to be stimulated to make 

 them active. 



5. The Development of Eggs. It was said a moment 

 ago that the egg is a cell. So it is, but there is another 

 fact that must be mentioned about it. It is not just like 

 the cells of the body from which it comes. In order to 

 make this clear we must recall a few facts about the 

 structure of cells. In each cell, aside from the more fluid 

 protoplasm, (cytoplasm) there is a definite protoplasmic 

 body called the nucleus. In this nucleus there is a very 

 important substance called chromatin. As the time 

 approaches for a cell to divide, this chromatin tends to 

 collect into a number of rod-like bodies known as 

 chromosomes. The number of these in the body cells of 

 a given species of plants or animals is practically constant. 

 These chromosomes behave in a very interesting way in 

 the development of an egg. 



Let us suppose that the nucleus of the ordinary cells 

 of an animal body that produces an egg has sixteen of 

 these chromosomes. By a process which we will not 

 study in detail here, the nucleus of the egg cell loses just 

 one-half its chromosomes as it prepares for fertilization. 

 In the supposed case the nucleus of the ripe egg cell 

 would contain eight chromosomes instead of the sixteen 

 which is characteristic of the other cells. 



This point you must keep in mind. We shall need it 

 to make clear what happens later. The cells of animal 

 bodies have twice as many chromosomes as their ripe 

 eggs have. 



6. Sperms and Their Characteristics. In many ways. 



