THE BREEDING OF CHICKENS 87 



approximately a central positiun. While the nucleus is still 

 at the center a deposit of extremely fine granules of ,yolk is 

 formed around it and ^-radually extends toward the cell wall. 

 This deposit is what is later recognized as the lateljra of the 

 flask-sha])ed mass of white yolk. 



When the ovum has reached a size approximating 0.66 

 mm., the nucleus migrates toa positionat the end of the flask- 

 shaped formation and just under the \'itelline membrane. 



Successive layers of yellow yolk are deposited around the 

 central mass of wdiite yolk by the activity of the peripheral 

 layer of protoplasm. These layers are somewhat correlated 

 with the periodic daily physiologic rhythm of the vitality 

 of the hen. 



When the yolk conies to full size, it escapes from the ovary 

 by the rupture of the follicle along the stigma, into the funnel 

 of the oviduct, which, as Lillie' states, "swallows it, so to 

 speak, and it passes on by peristaltic contraction of the ovi- 

 duct. The escape of the ovum from the follicle is known as 

 the process of ovulation." 



"The stinmli which initiate the peristaltic action of the 

 oviduct and attract the funnel to the mature follicle are 

 among the unsolved problems of physiology. The yolk is 

 already oriented in the follicle before the funnel encloses it."* 

 " Internal pressure due to continued yolk formation is prob- 

 ably the most important factor in the normal rupture of the 

 follicle, since closing the funnel or removing the duct ai)par- 

 ently does not greatly delay ovulation."^ 



Fertilization takes place almost immediately after o\ula- 

 tion, tiie spermatozoa having made their way through the 

 entire length of the oviduct. Lillie* states that "The ovum 

 is surrounded immediately after ovulation . . . bj- a 

 fluid containing spermatozoa in suspension. The entrance 

 of se\eral spermatozoa (into the ovum) seems to be char- 

 acteristic of vertebrates with large o\"a." 



Only one of these (spermatozoa) serves as a functional 

 sperm nucleus: the remainder or supernumerary sperm 



' Oppos. cit. 2 Curtia, Maine Bulletin No. 228. 



^ Pearl and Curtis, Journal Experimental Zoology, vol. vii, N(.) 3. 

 * Oppos. cit. 



