CHAPTER I 



THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 

 THE GERM CELLS 



THOSE animals, whose offspring reach maturity with reasonable surety 

 (as the result of internal fertilization and postnatal care), produce fewer 

 germ cells, especially ova, than those that leave fertilization to chance and 

 development to hazard. The codfish produces 10 million eggs in a breeding 

 period, a sea urchin 29 million; in certain mammals and birds only a single 

 egg is matured, yet the stock of each remains constant. 



The highly differentiated human organism, like all other vertebrates 

 and mo'st 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 

 produced in the ovary. It is nearly .spherical in form and possesses a 

 nucleus with nucleolus, chromatin network, and nuclear membrane (Figs, 

 i and 2). 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 cytoplasm, of 

 the ovum is distinctly granular, containing more or less numerous yolk 

 granules, mitochondria, and rarely a minute centrosome. The yolk gran- 

 ules, 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 the uterine wall of the mother and is nourished by it. A much 

 larger amount occurs in the ova of fishes, amphibia, reptiles, birds, a"ncf 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, such as the zona pellucida. In lower vertebrates tertiary 

 membranes may be added as the egg passes through the oviduct and uterus 

 the albumen and shell of the hen's egg (Fig. 3) or the jelly of the frog's 

 egg are of this type. 



The human ovum is of relatively small size, measuring from 0.22 to 

 0.25 mm. in diameter (Fig. i). The cytoplasm is surrounded by a thick, 



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