Gonadal Cycles and Gamete Production
According to the germinal line theory (Weismann 1885), metazoan organisms can be reduced to two components, the soma and the germinal line. The soma constitutes the bulk of the body, has a limited life span and is the part which dies; the germinal line is potentially immortal. It is represented by the primordial sex cells and by the male and female gametes which arise from them and link the successive generations of a species into a continuous lineage. The germinal line is often recognisable early in embryological development. The primordial germ cells of Chrysemys picta arise in a posteriorly directed horseshoe-shaped zone of the extra embryonic hypoblast (endoderm); they are distinguishable from the surrounding endodermal cells by their large size, and the fact that they contain yolk granules; they migrate interstitially, by active, independent, amoeboidal movement to the embryonic anlagen of the gonads, the germinal ridges (Allen 1906). Once the primordial germ cells of chelonians are grouped in the hypoblastic crescent that surrounds the caudal extremity of the embryo, they reach the germinal ridges by amoeboid movements through the splanchnopleure and lining of the digestive tract (Hubert 1985). The primordial germ cells, or the oogonia and spermatogonia arising from their mitotic divisions, remain dormant, often for long periods, until the organism reaches reproductive maturity. Only then do they undergo meiosis to become gametes — the eggs and spermatozoa.
KeywordsSertoli Cell Germinal Vesicle Sperm Storage Germinal Epithelium Follicular Atresia
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