In nature it is much like the conjugation described for 

 the lower plants and animals, and may have been 

 developed from it. When the full union of the sperm 

 with the egg has been finished, we have a well nourished 

 cell with a nucleus made up of two parts, coming equally 

 from a male and a female cell. The protoplasm and food 

 of this cell, on which further growth is supported, come 

 almost entirely from the female cell. 



The make-up of this new nucleus is peculiarly 

 interesting and important. The student will recall that 

 the nucleus of the egg, as it ripened, lost one-half of its 

 chromosomes. That is to say, if 2X is the number of 

 chromosomes found in the cells of the body in a given 

 species, these are reduced to X in the egg. The same is 

 true of the sperm nucleus. Therefore, when these two 

 bodies unite, they bring to the union two nuclei of just 

 one-half value so • far as chromosomes are concerned. 

 When they unite the nucleus is completely restored to 

 the proper number of chromosomes. Whatever was lost 

 has been restored; and, instead of being from one source 

 only, the new cell has material in it usually coming from 

 two individual and distinct parents. It is now known as 

 a fertilized egg. 



Recall that the forming of the fertilized egg is not 

 reproduction. Reproduction gave the egg and the sperm 

 cell. They were the offspring of their respective parents, — 

 two quite different kinds of offspring. The eggs and 

 sperms were male and female individuals. They were so 

 feeble that both of them would have died but for their 

 union. When they unite there is only one individual where 

 there were two before. This is just the opposite of 

 reproduction, which gives two individuals where there 

 was only one before. But this new individual has powers 

 of development which neither of the cells that entered 

 into it had. Instead of being feeble, as these two cells 

 were, it is completely renewed. It has youth and the 

 power to grow into an adult of the species to which it 

 belongs. 



A fertilized egg, then, is the result of a union of two 

 individuals from different sources. Each adult is not one 

 offspring merely. It is a composite made up of two 

 offspring, a male and a female. 



6. Fertilization and "Blood." We have a saying that 

 "blood will tell," and we mean by it that good or bad 

 qualities in the parents are sure in the long run to crop 

 out in their descendants. The reasons for this always 

 interest us. This wonderful process of fertilization throws 

 some light on the fact of inheritance. We know that 

 everything that is actually inherited, in the biological 

 sense, comes by way of these sex cells. To be sure, 

 parents can influence and change their offspring in other 

 ways than this, but such influence is not inheritance. 



Now if this is true, we can make some valuable 

 conclusions from what we have. All the protoplasm 

 outside the nucleus came from the mother; the matter 

 in the nucleus, and only that, came equally from both 

 parents. But so far as we can see, in the long run, 

 offspring are no more liable to be like the mother than 

 like the father. These facts show pretty conclusively 

 that those qualities in which the parents are unlike are 

 carried somehow by the nuclei, because only the nuclei 

 come equally from both parents. It is clear that likeness 

 to one parent or the other is not determined by bulk of 

 protoplasm; otherwise the female characteristics would 

 be much more likely to be transmitted than those of the 

 male. It is believed by many students of the subject 

 that they are carried by the chromosomes which are so 

 equally shared by the male and female in fertilization 

 (female, X plus; male, X equals fertilized egg, 2 X). 



7. Artificial Fertilization. The process described in 



this chapter varies considerably in different species of 

 animals and plants, but what has been said will serve to 

 give you an idea of what occurs in the great majority of 

 eggs and sperms that develop into adult plants or animals. 

 Usually an egg dies and disintegrates if it is not fertilized 

 by a sperm nucleus. However, a most wonderful discovery 

 was made only a few years ago. Some eggs of sea- 

 urchins were experimented with. Now the eggs of sea- 

 urchins, so far as we know, never de^-elop without being 

 fertilized. But a scientist took such eggs directly from 

 the ovary, where they could not have been exposed to 

 germs, and by putting them in water of a different density 

 from that of ordinary sea water and then returning them 

 to the sea water, he succeeded in ha: ing them begin to 

 develop, instead of die and decompose as they would 

 naturally do without fertilization. In other words, by 

 outside chemical stimulation, eggs were "artificially 

 fertilized," so that they began to develop without uniting 

 with sperms. This work has been repeated many times 

 since, and we find that the same thing is possible with 

 many different kinds of eggs; of worms, mollusks, and 

 even of vertebrates. It is also found that numbers of 

 other chemical substances and even other kinds of stimuli 

 may so arouse these eggs as to start development. All 

 of this suggests that the work of the sperm is to act as 

 a stimulant to the sluggish egg, and cause it to become 

 active in development, when it could not do so unaided. 



CHAPTER TWELVE. 

 DIFFERENT KINDS OF PARENTS. 



1. Review of Beginning of Sex in Offspring. We have 

 seen that the striking differences between eggs and sperms 

 are rather constant for all higher organisms. These 

 differences are the first signs of sex; indeed they are the 

 most important elements in sex. We call the egg a 

 female gamete and the sperm a male gamete. It is 

 important to remember that the gametes are the offspring 

 in reproduction. They do not merely unite and develop 

 into offspring. When they are produced the parent has 

 "reproduced." Reproduction consists in the formation 

 and the separation of these male and female cells from 

 the parent. All the other facts of sex with which we are 

 familiar come from this starting point. 



2. Simplest Cases in Which Eggs and Sperms are 

 Found. You remember that there are no eggs and sperms 

 in the simplest form of reproduction. There we see 

 merely fission or budding from one parent, with no union 

 of any kind. The simplest unions we have discovered 

 are those of conjugation, in which two similar cells, often 

 from the same parent, unite. From this we pass gradually 

 to different kinds of gametes which regularly unite. In 

 some cases eggs and sperms are produced by the same 

 mdividual. They may be produced at the same time or 

 at different times. A parent which produces both male 

 cells and female cells cannot rightly be called either male 

 or female. It is called an hermaphrodite. The word is 

 derived from Hermes and Aphrodite, Greek male and 

 female gods. 



Many of the lower plants, and some of the simpler 

 animals produce both kinds of gametes in the same 

 individual. In such a plant as Vaucheria, or Oedogonium 



