CHAPTER I 

 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



THE GERM CELLS 



The human organism with its various tissues composed each of aggregations 

 of similar cells is, like that of all other vertebrates, developed from the union 

 of two germ cells, the ovum and spermatozoon. 



The Ovum.— The female germ cell or ovum is a typical animal cell pro- 

 duced in the ovary [for structure of typical cell see histologic texts]. It is nearly 

 spherical in form and possesses a nucleus with nucleolus, chromatin network, 

 chromatin knots, and nuclear membrane (Fig. i). The cytoplasm of the ovum 

 is distinctly granular, containing more or less numerous yolk granules and a 

 minute centrosome. The nucleus is essential to the life, growth, and reproduction 

 of the cell. The function of the nucleolus is unknown; the chromatin probably 

 bears the hereditary qualities of the cell. The yolk granules, containing a fatty 

 substance termed lecithin, furnish nutrition for the early development of the 

 embryo. A relatively small amount of lecithin is found in the ova of mammals, 

 the embryo developing within, and being nourished by, the uterine wall of the 

 mother. It is much larger in amount in the ova of fishes, amphibia, reptiles, 

 birds, and the primitive mammalia, the eggs of which are laid and develop 

 outside of the body. The so-called yolk of the hen's egg (Fig. 2) is the ovum 

 proper and its yellow color is due to the large amount of lecithin which it con- 

 tains. The albumen, egg-membrane, and shell of the hen's egg are secondary 

 envelopes of the ovum. 



The human ovum is of small size, measuring from 0.22 to 0.25 mm. in diam- 

 eter (Fig. 1 A) . The cytoplasm is surrounded by a relatively thick radially striated 

 membrane, the zona pellucida. The striated appearance of the zona pellucida 

 is said to be due to fine canals which penetrate it and through which nutriment 

 is carried to the ovum by smaller follicle cells during its growth within the ovary. 

 The origin and growth of the ovum within the ovary are known as oogenesis, and 

 will be described in Chapter VIII. We may state here that each growing ovum 

 is at first surrounded by small nutritive cells known as follicle cells. These increase 

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