MATURATION. 23 



them also divides like the other chromosomes, each of the resulting sperma- 

 tids receiving one-half (Fig. 13, #). 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. 



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

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

 contain 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 containing 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 sex. 



The presence of accessory chromosomes has been demonstrated in many 

 invertebrates, especially insects. They have also been described in several 

 vertebrates such as the rat, fowl, guinea-pig, and even man. In many cases 

 the accessory chromosome of the male germ cell has a mate which differs, 

 however, in some way (size, appearance, etc.) and is designated the Y- 

 chromosome. An ovum fertilized by a spermatozoon containing the Y- 

 chromosome will give rise to a male; if fertilized by one containing the 

 X-chromosome the egg will develop into a female. 



There are many cases, particularly among parthenogenetic forms, where 

 sex cycles arise, which cannot be explained by chromosomal behavior. In 

 these cases nutrition seems to play an important part in determining the 

 : sex of the individual. But as to the great majority of forms investigated, the 

 weight of evidence supports the view that the chromosomes are the chief 

 agents in sex determination. 



Ovulation. 



Ovulation is the discharge of the ovum from the ovary, whether in the 

 human female or any of the lower animals. Our attention will here be con- 

 fined to the phenomenon as it occurs in mammals. 



Before the ovum escapes from the ovary it is contained in a structure 

 known as the Graafian follicle, which consists of a wall of epithelium, the 

 granular layer, enclosing a space filled with a viscid fluid, the follicular fluid. 

 Surrounding the follicle is a special layer of connective tissue, the theca fol- 

 liculi, which is a part of the ovarian stroma and contains many small blood 

 vessels. The egg cell is situated within a thickened portion of the epithelial 

 wall, the germ hill. The growth of the follicle itself will be described in the 

 chapter on the geni to-urinary system. 



