第八课 翻译.doc

Production of Sperm and Eggs In sexually reproducing organisms males and females produce sex cells, known as gametes. These are swimming sperm in males and ova (eggs) in females. 在有性繁殖生物中，雄性和雌性产生称为配子的性细胞。配子在雄性中是游动的精子，在雌性中是卵。 The process of sperm production, spermatogenesis, takes place in testes. The sperm originate in gonial cells (spermatogonia) in the walls of seminiferous tubules. Spermatocytes produced by mitosis in spermatogonia divide meiotically to generate haploid spermatids. The mature sperm has a tail, a nucleus containing haploid chromosomes, and a front end with an acrosome, the storage site for enzymes that will aid fertilization. 精子产生的过程发生在睾丸中。精子发生在输精管壁的性原细胞（精原细胞）中。在精原细胞中经有丝分裂产生的精母细胞减数地分裂产生单倍体精细胞。成熟精子有一个尾巴，含有单倍染色体的核和具有一个顶体的前端，顶体是贮藏帮助受精的酶的地方。 Ova, which are produced during oogenesis, are generated in gonial cells (oogonia) of the females ovaries. Oocytes then enter a stage of arrest in early meiosis. At a species-specific later point, a final ripening (ovulation) and the first meiotic division occur. A second meiotic division, followed by development of the embryo, takes place if the egg is fertilized. 在卵发生中产生的卵被形成在雌性卵巢的性原细胞（卵子细胞）。然后，卵母细胞进入早减数分裂滞育时期。在特殊种稍迟点，最后成熟（排卵），第一次减数分裂发生。如果卵受精，随着胚的发育，第二次减数分裂发生。 Eggs vary greatly in size from species to species and have complex structures. Virtually all developing animal ova are surrounded by helper cells, either follicle cells or nurse cells. Depending on the species, eggs also store varying amounts of yolk, a reservoir of nutrients produced by digestive-gland cells in the mothers body. Finally, follicle cells or cells of the maternal oviduct provide protective coatings for the egg, including albumen (egg white) and various types of outer membranes and shells. 在不同种类间，卵在体积上变化很大，并且有复杂的结构。事实上，所有发育的动物卵都被助细胞、或者滤泡细胞或者滋养细胞所包围。根据种的不同，卵也贮存不同量的卵黄，卵黄是在母体中由消化腺细胞产生的营养库。最后，滤泡细胞或母体的输卵管细胞为卵提供保护层，包括蛋白（蛋白）和不同类型的外膜和壳。 Frog oocytes have served as model systems for studies of oocyte development. During maturation they produce hug