410 EMBRYOLOGY OF THE CHICK. 



about 1 mm. in size. A little later each ovum is enclosed in 

 two membranes — the inner a radiately striated membrane called 

 the zo7ia radiata; the outer one lying between the zona and the , 

 epithelial layer surrounding the ovum, the vitelline membrane. 

 The zona disappears, and leaves the ovum surrounded by the 

 vitelline membrane alone. The largest of the capsules then 

 contains a roundish yellow yolk-like body enclosed in a thin 

 membrane — the ovarian ovum. Upon this body 'wiU be observed 

 a small disc, the germinal disc, vrhich consists of a globular 

 body, the germinal vesicle, embedded in a mass of protoplasm. 

 This yolk mass is then the true ovum, and this alone. When 

 quite ripe it is dehisced from the ovary and passed into the 

 funnel-like head of the oviduct. The reader must again be 

 referred back to p. 344, where the structure of the oviduct is 

 pointed out. It is in this oviduct that the vs^hite, shell, &c., of 

 the egg is formed, by secretions from its glandular walls. In 

 the second part of the tube, the oviduct proper, the albumen is 

 deposited, and then the chalazee and the albuminous layers com- 

 pleted. At the lowest end of this second division the shell- 

 membrane is produced. It is said that the egg remains in the 

 oviduct proper about three hours. The shell is formed by the 

 walls of the third portion of the oviduct, the uterine portion. 

 From the glandular walls of this uterine tube there is poured 

 out a thick white covering to the egg, in which the inorganic 

 matter is deposited. It takes normally from fifteen to twenty 

 hours to traverse this part of the egg-tube. On leaving it the 

 egg is rapidly passed out of the cloaca by the muscular contrac- 

 tions of the uterus, &c. Thus by knowing the structure and 

 function of the various parts of the oviduct, we can tell what 

 portion is diseased when we find parts of the egg ill-formed, 

 such as absence of white or a thin shell. 



The egg is fertilised in the first part of the oviduct, where 

 the spermatozoa may be found freely moving about in the 

 fluid contents of that cavity. After the egg is fertilised, 

 and the various phenomena directly following impregnation 



