574 



COLLEGE ZOOLOGY 



other are spoken of as being homologous. 

 The zygote and its cellular descendants all 

 contain the number of homologous pairs of 

 chromosomes characteristic of the species. 



The formation of eggs 

 and sperms 



Obviously, if the sperm and egg which 

 combine to form the zygote contained the 

 same number of chromosomes as somatic 

 (body) cells, the number of chromosomes 

 present in a species would double every 

 generation. However, during the formation 

 of eggs and sperms, an orderly process known 

 as maturation (meiosis) prevents this dou- 

 bling from occurring. The most important 

 single effect of maturation (Fig. 404), is 

 a reduction of the number of chromosomes 

 in eggs and sperms to one-half the number 

 contained in somatic cells. Though the de- 

 tails of this reduction are variable in differ- 

 ent species and somewhat complicated in 

 all species, the process can be described 

 simply as a single duplication of each chro- 

 mosome, followed by two nuclear divisions. 



The two divisions in maturation are 

 spoken of as the reductional and equational 

 divisions for reasons which will become clear 

 in what follows. Early in the first or reduc- 

 tional division, each chromosome duplicates 

 itself to form two chromatids (Fig. 10), 

 which do not separate until later in matura- 

 tion. 



A second important process, synapsis, also 

 occurs in the primary oocyte or spermato- 

 cyte. Synapsis describes the process whereby 

 the two members of a homologous pair of 

 chromosomes come to lie very close to each 

 other. Because a synapsed pair of chromo- 

 somes is composed of four elements, the 

 two chromatids of each duplicated chro- 

 mosome, the whole structure is called a 

 tetrad (Fig. 404). In synapsis, the chromo- 

 somes may lie so close together that under 

 the microscope the tetrad may appear as a 

 single dense unit. During synapsis, exchanges 

 may occur between the chromatids of the 



two homologous chromosomes, a phenome- 

 non called crossing over, to be discussed in 

 a later section. While the chromosome pairs 

 are still in synapsis, they migrate to the 

 equatorial plate of the first division spindle. 

 When they arrive, they become oriented in 

 such a way that one member of each pair 

 is directed toward one pole of the spindle, 

 the other member toward the opposite pole. 

 The first division is then completed by the 

 separation of the homologous members of 

 each pair (separating black from white, in 

 the diagram Fig. 404). This is a reduction 

 division. Note that the chromosomes in 

 each of the secondary oocytes and spermato- 

 cytes are not an assortment of any combina- 

 tion of chromosomes, but a very orderly 

 group consisting of one chromosome from 

 each pair of homologous chromosomes con- 

 tained in the parental cell (primary sperma- 

 tocyte or oocyte). In other words, while a 

 somatic cell contains two chromosomes of 

 each kind (similar in length, form, and 

 genie content) to be found in a species, a 

 secondary oocyte or spermatocyte will con- 

 tain only one of each kind. For example, 

 human somatic cells contain 46 chromo- 

 somes (23 pairs) whereas a human second- 

 ary oocyte contains 23 (one from each 

 pair). 



The second division, the equational divi- 

 sion, may follow closely on the first. The 

 essential feature of the second division is 

 extremely simple. You will recall that fol- 

 lowing duplication, each of the chromo- 

 somes of the first division was made up of 

 two sister chromatids. In the second divi- 

 sion, every chromosome becomes oriented 

 on the two spindles. The sister chromatids 

 then separate from each other (black from 

 black and white from white in Fig. 404). 

 The separated chromatids are known as 

 chromosomes. 



Following maturation, the immature egg 

 or sperm (spermatid) matures to become a 

 mature sperm or egg, but there is no further 

 chromosomal change. When a mature egg is 

 fertilized by a sperm, the zygote resulting 



