REPRODUCTION AND DEVELOPMENT IN CHORDATES 



differentiation occurs in the female germ cell, or primary oocyte, as it is now 

 called. In all female vertebrates certain of the undifferentiated germ cells 

 form a layer around the primary oocyte. This envelope, or follicle, serves 

 a nutritive function during the storage of food, usually in the form of yolk, 

 which occurs in the oocyte during the growth period. The follicles of 

 different vertebrates vary in thickness; those of mammals are very large 

 (Fig. 5.2 and S.3B). Not all vertebrates store a large supply of food for the 

 young animal that may develop from the egg if it is fertilized. ITie method 

 and place of development are correlated with the amount of food stored in the 

 female germ cell and will be discussed later. When food storage is complete, 

 the primary oocyte divides to form two cells very unequal in size. There is 

 a large cell, the secondary oocyte, containing most of the food, and a very 

 small cell, the first polar body, which has only a thin layer of cytoplasm 

 around its nucleus. The division which produces these cells is the first 

 meiotic or disjunctional division, and the nucleus in each cell contains half- 

 tetrads, or dyads, just as does the nucleus of each of the secondary sperma- 

 tocytes. At the second meiotic or equational division the first polar body 

 divides to form two polocytes, or polar cells, of equal size, but the secondary 

 oocyte gives rise to a small cell, the second polar body, and to the ovum, or 

 mature female germ cell, which contains most of the food that was stored in 

 the primary oocyte. The three polar cells and the ovum have comparable 

 nuclei; each contains one chromosome corresponding to each homologous pair 

 in the oogonia and somatic cells. Only one ovum, or macrogamete, is pro- 

 duced from each primary oocyte. The small polar cells are non-functional 

 and die. 



All the cells of the vertebrate organism become differentiated to perform the 

 activities of the particular organs of which they form a part. Specialization 

 of the germ cells occurs much later in life than differentiation of most of the 

 somatic cells and is limited to the period of sexual activity in the organism. 

 Primordial germ cells are totipotent cells (p. 159). The essential difference 

 between somatic cells and the germ cells is to be referred to their activities: 

 the somatic cells are specialized in ways which contribute to the maintenance 

 of the life of an individual, whereas differentiated germ cells make possible 

 the reproduction of a new individual and so provide for continuity of the 

 species. Both groups of cells have the same essential structure of nucleus and 

 cytosome; both have the same requirements for life. Somatic cells contribute 

 to the existence of the germ cells of the same generation, whereas germ cells 

 make possible the existence of somatic cells of the succeeding generation. 



Reproduction 



Historical, Since Aristotle's observations, in the fourth century B.C., on 

 the developing hen's egg, students have been interested in the origin of new 

 individuals. Before the invention of the microscope the male germ cells could 



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