GENERAL ZOOLOGY 



Fig. 5.4. The development of the hen's egg. A, portion of the ovary, showing oocytes in 

 different stages of growth. B, the egg at the time of laying, with part of the shell removed 

 to show the yolky zygote (the "yolk") suspended by denser cords of albumen (the "white"); 

 the small disk of cytoplasm called the blastodisk is shown on the surface of the zygote. C, 

 the blastodisk removed from the egg, to show its cleavage; after cleavage it is called the 

 blastoderm. D G, successive stages in the origin of the embryo and the spread of the margin 



reptiles. Other mammals retain the fertilized eggs during development, and 

 the oviducts exhibit conspicuous modifications related to retention and nour- 

 ishment of the embryo or developing young individual. These mammals are 

 viviparous. The ostial portion of each oviduct is slender and is known as the 

 Fallopian tube, or uterine tube (Fig. SAB). Posterior to the Fallopian tube, 

 the oviduct expands to form the uterus, which is the region where the 

 embryos develop, and terminates in a short vagina, which opens into the 

 urogenital sinus. Although primitive mammals have a pair of oviducts, in 

 higher forms there is a progressive fusion beginning with the vaginal portions, 

 and in man the Fallopian tubes are the only paired parts (Fig. 5.5). 



Vertebrates are typically either male or female, but they pass through a 

 developmental stage in which they are sexually indifferent; that is, it is im- 

 possible at such a stage to determine whether the primordial gonads are 

 ovaries or testes, and the primordial reproductive ducts of both sexes are 

 present. During later development the gonads differentiate into either ovaries 

 or testes; the correlated ducts develop, and those of the opposite sex degen- 

 erate. In the adult males of some fishes and frogs, however, the oviducts 



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