158 POUI.TRY DISEASES AND THEIR TREATMENT. 



From this layer are formed the chalazae, or cords of twisted, 

 thickened albumen, at each pole of the yolk. 



11. Yolk, around which albumen is being secreted. 



12. The germinal disc. This is the living portion of the egg, 

 from which the future chick develops, the main mass of yolk 

 serving as food material for the developing embryo during the 



process. 



13. Anterior end of the isthmus of the oviduct. The prim- 

 ary function of the isthmus is to secrete about the egg the shell 

 membrane, the dense white membrane closely adherent to the 

 inside of the shell of an egg. 



14. The uterus, or shell gland, in which the shell is put on 

 the egg. 



15. The rectum. 



16. The walls of the abdomen cut and folded back. 



17. External opening of the cloaca, or common space into 

 which open (a) the rectum, (b) the oviduct and (e) the ureters, 

 or kidney ducts. 



The processes concerned in the foi'mation of an egg are thus 

 summarized by Lillie (The Development of the Chick, New 

 York, 1908, pp. 23-25) : 



"The formation of an egg takes place as follows : The yolk, 

 or ovum proper, escaped by rupture of the follicle along a pre- 

 formed band, the stigma (fig. 40), into the infundibulum, 

 which swallows it, so to speak, and it is passed down by peris- 

 taltic contractions of the oviduct. The escape of the ovum 

 from the follicle is known as the process of ovulation. During 

 its passage down the oviduct it becomes surrounded by layers 

 of albumen secreted by the oviducal glands. The shell-mem- 

 brane is secreted in the isthmus and the shell in the uterus 

 (fig. 40). The ovum is fertilized in the uppermost part of the 

 oviduct and the cleavage and early stages of formation of the 

 germ-layers take place before the egg is laid. The time occu- 

 pied by the ovum in traversing the various sections of the ovi- 

 duct is estimated by Kolliker as follows : Upper two-thirds of 

 the oviduct about 3 hours (formation of albumen), isthmus 

 about 3 hours (secretion of shell-membrane), uterus 12 to 24 

 hours (formation of shell and laying). These figures are only 

 approximate and it is obvious that they are likely to vary con- 

 siderably in different breeds of hens." 



