as we have seen, the sperms are opposite in their nature 

 from the eggs. The sperms are always mihute, even 

 more minute than the smallest eggs. Animal sperms are 

 almost all quite active, with an active tail or flagellum 

 that drives them through the fluid in which they occur. 

 Sperms are real cells, however. They have a perfect 

 nucleus, and a delicate supply of protoplasm, most of 

 which is in the tail mentioned above. Sperm cells vary 

 in shape rather more than eggs. Usually a species 

 produces many more of the small sperms than of the 

 large ova. 



7. The Development of Sperms. In development the 

 nuclei of sperms of animals behave just as those of the 

 eggs. If the body cells have sixteen chromosomes in 

 their nuclei, these divide, as sperms are being formed, 

 in such a way that each sperm nucleus gets just eight. 

 As the biologist says it, there is a reduction division in 

 the forming of eggs and sperms by which they get only 

 X chromosomes when there are 2 X chromosomes in the 

 cells of the body of the parent organism. The mother 

 cells of the sperm are not large cells to start with. The 

 divisions that produce the sperms take place rapidly and 

 the protoplasm does not grow much in the meantime. 

 Hence the sperms cannot fall heir to much protoplasm, 

 and so are small. 



8. The Reduction Division in Plants. What has just 

 been said about the reduction division in animals does not 

 hold true in forming eggs and sperm in plants. It is true, 

 however, that plants, like animals, do have in their life 

 history a reduction in the number of chromosomes which 

 compensates for the doubling of this number which occurs 

 when sperm and egg fuse. 



Plants differ from animals in having what is called 

 alternation of generation, a phenomenon described in a 

 later chapter. One of these generations (gametophyte() 

 bears gametes, the other (sporophyte) bears spores, and 

 the two together complete the life cycle of the plant. 

 The reduction division occurs in connection with the 

 formation of the spores. 



9. Sex in the Offspring. The differences between the 

 eggs and sperms are not just occasional, exceptional 

 facts. They are very constant differences in the plant and 

 animal kingdoms. Animals and plants regularly produce 

 two kinds of offspring, eggs and sperms; usually neither of 

 these can develop alone; they unite into one individual, 

 and this individual has all the powers of the species. These 

 differences in the gametes are differences of sex. They 

 are the most fundamental differences in sex. The eggs 

 and sperms are the "sex-cells." The egg is a female cell, 

 or offspring; the sperm is a male offspring. Fertilization 

 produces a new individual made,up/of the two kinds of cells. 



CHAPTER XI. 



FERTILIZATION. 



1. Review of Conjugation. In an earlier chapter a 

 study was made of the union of gametes that are wholly 

 similar. This sort of union is called conjugation, and is 

 quite common among the lower plants and animals. In 

 these gametes there is no outward sign of the differences 

 which we think of as belonging to males and females. 

 We now wish to study the union of eggs and sperms. 



such as were described in the preceding chapter, where 

 there is a very striking difference in the gametes. These 

 differences are sex differences and we call the offspring 

 sex-cells. Their union we call fertilization. 



2. The Effects of the Special Developm«nt of Eggs and 

 Sperms. In Chapter X it was shown that with all their 

 differences, eggs and sperms agree in one important 

 particular. In both cases, when ripe, they have in their 

 nuclei only one-half the number of chromosomes usual in 

 the cells of the species. There are some interesting results 

 that seem to come from this reduction in the chromosomes. 

 Under ordinary circumstances neither of these cells has 

 the power of dividing again, or of developing any further 

 in any way._ There are exceptions to this, but it is true as 

 a general thing. In some of the gametes which are alike 

 we saw that they might resume development separately if 

 they did not happen to unite. In the case of eggs and 

 sperms, unless they unite, both will die. Union, then, has 

 become almost imperative in these special cells. The sperm 

 is too used up to divide; and the egg has too much 

 protoplasm to be stimulated into division by the small 

 amount of nuclear material in it. They do not behave as 

 young cells. They are like two run-down batteries. 



In the second place, these cells are very different in 

 their structure and development. The egg with its large 

 size and rich nutrition is passive and sluggish, while the 

 sperm is reduced to the very lowest living terms. Both 

 these cells are essential to the on-going of the species. 

 Their very differences, therefore, will make it absolutely 

 sure that the powers they have and the work they do 

 must be very different. 



Eggs develop much more slowly, sometimes requiring 

 years; are usually produced in much smaller numbers; 

 have no ability to take an active part in the mating, but 

 secrete substances that attract the male cells. The 

 sperms, on the contrary, are actively sensitive to the 

 substances secreted by the egg or its surrounding tissues, 

 are produced rapidly and in enormous numbers, swim 

 actively about and find the female cells. It is thus seen that 

 their powers and behavior are strongly complemjentary 

 and that they are wonderfully adapted to each other: 

 passive and motile, secretion and sensitiveness, attraction 

 and responsiveness, large amount of food substances in 

 protoplasm and almost no protoplasm at all. This 

 adjustment between male and female gametes is the most 

 fundamental fact of sex and mating, and helps determine 

 all the other facts to which we shall give attention later. 



3. Method of Fertilization. In the union of the sperm 

 with the egg there are two distinct problems. One is the 

 actual task of uniting these two cells into one when they 

 have come into the same region. The other is that of 

 bringing the sperms and eggs into such close range that 

 they can have any chance of uniting. This latter problem 

 must be solved by the parents that produce the cells, and 

 is solved in very different ways in different plants and 

 animals. It will be considered in the next chapter. 



As has been suggested above, the eggs have much 

 attraction for the sperm when they come near to one 

 another. The sperms move toward the egg, attach 

 themselves to the surface, and one of them (usually only 

 one) succeeds in penetrating the egg membrane. The 

 sperm enters the protoplasm of the egg. When it is in 

 the protoplasm of the egg it looks like a new nucleus. The 

 egg also has its own nucleus; hence the egg is now a cell 

 with two nuclei in it. These two nuclei exert an attraction 

 for each other; they gradually move together through 

 the protoplasm, and finally fuse. 



5. The Union of Egg and Sperm: Its Results. The 

 process described above is called fertilization and is 

 found in practically all the higher animals and plants. 



