44 POULTRY DISEASES 



line the stigma where the follicular wall is becoming tliin 

 preparatory to discharging the yolk into the oviduct. The 

 discharge of the yolk into the oviduct is sometimes spoken of 

 as ovulation. 



The yolk has its origin in a minute sphere containing a nu- 

 cleus, as illustrated by a, Fig. 14. This nucleus marks the 

 point of the development of the embryo chick after fertiliza- 

 tion. It is noted to be located in the central portion. When 

 the cell begins the development of the yolk there is noted 

 first a deposit of fine granules of yolk around the central nu- 

 cleus. These granules of yolk material gradually extend to- 

 wards the cell Avail. This deposit is known as the latebra or 

 the flask-shaped mass of white yolk forming thin layers of 

 yellow yolk. 



Later, when the ovum has reached a size of about 0.66 milli- 

 meter in diameter, the nucleus occupies a position just under 

 the vitelline membrane and at the end of the flask-shaped mass, 

 as illustrated in a, Fig. 14. 



Later there are formed several layers of yellow yolk depos- 

 ited around the central mass of white yolk, apparently brought 

 about through the secretion of the peripheral layer of proto- 

 plasm. 



The spermatozoa, a, Fig. 18, make their way by aid of their 

 terminal flagella or tails through the oviduct and fertilization 

 takes place as soon as the yolk has entered the oviduct. Only 

 one spermatozoon is utilized in this fertilization process. The 

 balance are repelled from the cell. 



After the yolk passes into the oviduct albumen is formed 

 around it in the second or upper portion by specialized col- 

 umnar epithelial cells. 



The contraction of the muscles of the oviduct forces the 

 contents along. AVhen the albumen formation is completed 

 the newly forming q^^ passes into the isthmus or third portion 

 where through the activity of other specialized cells a mem- 

 brane is formed around the mass. 



In the lower portion, as illustrated in Fig. 13, letter /, the 

 calcium layer or shell is formed to protect the delicate mass 

 within from external violence. Here the tint or color is pro 

 duced in shells other than white. 



The formation of the allnimen around the yolk in the up- 

 per portion of the oviduct is probably accomplished in about 

 three hours. The membrane surrounding tlie Q^g, mass is 

 formed in the isthmus in a])out the same length of time. The 

 formation of the shell and the expulsion of the egg will be 

 accomplished in from twelve to eighteen hours. 



