558 



CONTINUITY OF LIFE 



stead of two that were present in the sper- 

 matogonium, in other words, the haploid 

 number. The spermatid becomes reduced in 

 size, develops a tail, and matures in other 

 ways to become a full-fledged sperm ca- 

 pable of fertilizing an egg. 



During this process it will be noted that 

 the maternal and paternal chromosomes 

 (indicated by black and white) may be 

 distributed in several ways so that by the 

 time they reach the spermatid there may 

 be any combination present. There is, how- 

 ever, always a complete set. The mature 

 sperm gets only one member of each homol- 

 ogous pair of paternal and maternal chro- 

 mosomes, never both. This chance distri- 

 bution of the members of each chromosome 

 pair is very important in the understanding 

 of the mechanics of heredity which we shall 

 study presently. 



Oogenesis 



The process by which eggs are formed 

 varies only slightly from the preceding ac- 

 count for sperms. The principal difiFerence 

 begins when the primary oocyte (compar- 

 able to the primaiy spermatocyte) divides. 

 Instead of forming two equal-sized sec- 

 ondary oocytes, it produces one large one 

 and one very small one, the latter being 

 known as the 1st polar body. This discrep- 

 ancy in size is owing to the fact that virtu- 

 ally all of the cytoplasm goes to one daugh- 

 ter cell in the single secondary oocyte. The 

 next division operates on the same principle 

 and results in a single large ovum and a 

 2nd polar body. Because of their scanty 

 cytoplasm, the polar bodies are non-func- 

 tional and soon disintegrate. The apparent 

 value in this unequal division is to conserve 

 the cytoplasmic contents in order to retain 

 sufficient stored foods for energy during 

 the early stages of embryonic development. 

 This is not necessary for sperms, because 

 once they have contributed their load of 

 chromosomes to the es;"; in fertilization their 

 job is done. Furthermore, millions of sperms 

 are needed to insure fertilization whereas 



only comparatively few eggs are necessary 

 to maintain the race. 



The zygote formed at fertilization tlius 

 possesses the full diploid number of chro- 

 mosomes and all subsequent divisions are 

 mitotic until the individual is formed. Then 

 some of its cells are set aside to undergo 

 meiosis in the production of gametes, which 

 again unite with others, and so the process 

 continues from generation to generation. 

 We have seen that in the formation of 

 gametes, the chromosomes may be dis- 

 tributed in a variety of ways. The greater 

 the number of chromosomes (genes), the 

 larger the variety. It is this perennial dis- 

 tribution which accounts for the great vari- 

 ation seen in all living things. 



SIGNIFICANCE OF SEXUAL 

 REPRODUCTION 



The most obvious fact about sexual 

 reproduction which has come from the 

 discussion of gametogenesis is the great 

 variety of germ cells that can be produced 

 by one individual. From each tetrad each 

 germ cell may receive either a maternal or 

 a paternal chromosome but not both. This 

 applies to all of the chromosomes, which 

 makes the possible combinations in animals 

 with numerous chromosomes almost infi- 

 nite. This is further complicated by the fact 

 that the chromosomes wind about each 

 other during synapsis and often exchange 

 genes, a phenomenon known as the cross- 

 over ( see p. 606 ) . This exchange may occur 

 in several places in each chromosome pair. 

 When these produce tetrads and eventually 

 germ cells, the chromosomes will not be like 

 the original maternal or paternal chromo- 

 some but a combination of both. On the 

 average, chromosomes will break and ex- 

 change eenes in about three places, which 

 means that the original mixing on the basis 

 of chromosome is increased by 2^, or 8 

 times. Let us consider, for a moment, the 

 possibilities in man: there are 24 chromo- 

 some pairs in the body cells and, of course, 



