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TEXT-BOOK OF EMBRYOLOGY. 



of chromosomes are formed but the odd chromosome, which can usually be 

 distinguished by its appearance, is left without a mate. At the first maturation 

 division this univalent chromosome does not divide but passes as a whole to one 

 of the two resulting cells, thus giving two kinds of secondary spermatocytes 

 (Fig. 17, 4 and 5, x). When the secondary spermatocytes divide, however, the 

 odd chromosome in one of them also divides like the other chromosomes, each 

 of the resulting spermatids receiving one-half (Fig. 17, 7 and 8, x). Thus two 

 kinds of sperms are formed in equal numbers, containing respectively eight and 

 nine chromosomes. The odd chromosome is also known as the accessory or 

 X-chromosome. 



Germinal 



epithelium 



FIG. 1 8. From section of human ovary, showing mature Graafian follicle ready to rupture. 



Kollmann's Atlas. 



In the ovum no such complication arises, there being two "accessory" 

 chromosomes which unite in synapsis. All the mature ova will therefore con- 

 tain nine chromosomes. As a result, there are two combinations possible when 

 the male and female sex cells unite : an ovum may be fertilized by a sperm con- 

 taining either eight or nine chromosomes. In the first case the somatic number 

 in the fertilized egg will be seventeen and the egg will develop into a male. In 

 the second case the somatic number will be eighteen and the resulting individual 

 will be a female. In the example given, therefore, the presence or absence 

 of the "accessory" or odd chromosome will determine the nature of the sex 

 produced. 



The presence of "accessory" chromosomes has been demonstrated in many 

 Invertebrates, especially Insects. Recently they have also been described in 



