246 THE BIOLOGY OF BIRDS 



and two oviducts, and the bird sometimes laid two eggs in a 

 day. 



Origin of Germ-Cells.— There is no doubt that the 

 primordial germ-cells of the bird are set apart from body- 

 making cells at a very early stage in embryonic development. 

 In the chick, according to Charles H. Swift (1914), the 

 primordial germ-cells appear between the endoderm and 

 the ectoderm in a crescent at the margin of the area pellucida, 

 anterior and antero-lateral to the embryo. They are seen 

 during the primitive streak stage, while the embryo has 

 only about three segments. Subsequently by amoeboid 

 movements they enter the mesoderm and the incipient blood- 

 vessels of the mesoderm. They are carried at first by their 

 own movements, and later by that of the blood, to all parts 

 of the embryo and vascular area. They remain generally 

 distributed in this way until the embryo has about twenty 

 segments or somites. They seem to concentrate in the 

 direction of the future reproductive organ, but this may be 

 in part more apparent than real, for they degenerate else- 

 where. 



The important general fact is that the primordial germ- 

 cells or sex-cells appear very early in the development of 

 the body of the embryo. According to Swift (1915) the 

 germinal epithelium which forms the foundation of the 

 reproductive organs is well-defined between the 8oth and 

 90th hours of incubation in the fowl. There are recognisable 

 primordial sex-cells in this germinal epithelium, and, as we 

 have mentioned, there are others outside its limits, some of 

 which at least are known to undergo degeneration. 



Multiplication or proliferation of cells occurs in the 

 germinal epithelium, and the result is the formation of " sex- 

 cords." These sex-cords of the first proliferation are 

 epithelial in character, and the primordial sex-cells, which 

 are present in them, do not seem to have anything to do 

 with their formation. But it is possible that some of the 

 germ-cells stimulate the epithelial cells around them to 

 activity. The cords of first proliferation become seminiferous 

 tubules in the testes, medullary cords in the ovary. When 



