230 GENERAL ZOOLOGY 



instead of the expected 8 chromosomes, only 4 appear, but each 

 of the 4 is a double chromosome. This pairing of the chromo- 

 somes is known as synapsis and takes place in the prophase 

 of the first maturation division. 



Meiosis. — Owing to constant differences in form and size, it 

 is possible to arrange the chromosomes of the presynaptic period, 

 or of the cleavage period, or of any somatic cell, into two series 

 of homologous chromosomes. In the hypothetical case repre- 

 sented in Fig. 139, the two series consist of 4 chromosomes each. 

 During the prophase of the first maturation division these pair off 

 according to form, size, or other characteristics to form double 

 or bivalent chromosomes. As shown in Fig. 139, during the first 

 maturation division, the members of the synaptic pairs undergo 

 disjunction and pass as whole chromosomes to opposite poles of 

 the spindle, so that each secondary spermatocyte or each second- 

 ary oocyte and the first polar body, receive 4 chromosomes. The 

 reduction of the chromosome number from the diploid number 

 8 to the haploid number 4 is known as meiosis. It is also spoken 

 of as a qualitative division. In the second maturation division, 

 which follows, each of the four chromosomes divides in the usual 

 way, with the result that each spermatid and each ootid and 

 second polar body receive 4 chromosomes. The second division 

 then is a quantitative division, such as takes place in any somatic 

 mitosis. 



If, as appears to be the case in some animals, the first matura- 

 tion division is a quantitative one, each component of each 

 synaptic pair being divided longitudinally, the second maturation 

 division becomes a reduction division; but whether reduction 

 occurs in the first or second maturation division, the final result is 

 the same, viz., the ootid, polocytes, and spermatids all receive 

 the haploid number of chromosomes. The diploid or zygotic 

 number is restored at fertilization by the union of the nuclei 

 of the ootid and the spermatozoon. Since the cells of the 

 embryo, and later the adult, are produced by cell division from 

 the fertilized egg, it follows that each cell in the body is provided 

 with the zygotic number of chromosomes, except the germ cells 

 of the postreduction stages. It should also be clear that the 

 zygotic chromosome complex consists of a maternal and a 

 paternal series, which separate in meiosis. These facts are of 

 utmost importance in understanding the mechanism of heredity, 



