CHAPTER I 



THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



THE GERM CELLS 



The highly differentiated human organism, Hke all other vertebrates and 

 most invertebrates, develops 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. It is nearly spherical in form and possesses a nucleus with 

 nucleolus, chromatin network, chromatin knots, and nuclear membrane (Figs. 1 and 

 2). The cytoplasm of the ovum is distinctly granular, containing more or less 

 numerous yolk granules and rarely 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 yolk 

 is found in the ova of the higher mammals, since the embryo develops within, 

 and is nourished by, the uterine wall of the mother. A much larger amount occurs 

 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. 3) is the ovum proper and its yellow color is due to the large 

 amount of lecithin which it contains. 



Ova become surrounded by protective membranes, or envelopes. The 

 vitelline membrane, secreted by the egg itself, is a primary membrane (Fig. 2). 

 The follicle cells about the ovum usually furnish other secondary membranes, 

 e. g., the zona pellucida. Tertiary membranes may be added as the egg passes 

 through the oviduct and uterus — the albumen, shell membrane, and shell of the 

 hen's egg are of this type (Fig. 3). 



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

 eter (Fig. 1) . 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 folUcle cells during its growth within the ovary. 



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