10 EARLY EMBRYOLOGY OF THE CHICK 



insemination has taken place meanwhile, the spermatozoa 

 (Fig. 2) make their way along the oviduct where for several 

 days they may remain alive and capable of performing their 

 function of fertilization. Penetration of the ovum by sperma- 

 tozoa takes place in the region of the oviduct near the ovary, 

 before the albumen and shell have been added to the ovum. 

 Coincidently the second polar body is extruded. Although in 

 birds normally several spermatozoa penetrate the ovum, only a 

 single one unites with the female pronucleus. The fusion of the 

 male and female pronuclei in fertilization initiates the develop- 

 ment of the embryo and the cleavage divisions are begun while 

 the ovum is passing through the oviduct toward the cloaca and 

 receiving meanwhile its accessory coverings. 



The Formation of the Accessory Coverings of the Ovum. 

 The albumen, the shell membrane, and the shell are non-cellular 

 investments secreted about the ovum by the cells lining the 

 oviduct. In the part of the oviduct adjacent to the ovary a 

 mass of stringy albuminous material is produced. This ad- 

 heres closely to the vitelline membrane and projects beyond 

 it in two masses extending in either direction along the oviduct. 

 Due to the spirally arranged folds in the walls of the oviduct, 

 the egg as it moves toward the cloaca is rotated. This rotation 

 twists the adherent albumen into the form of spiral strands pro- 

 jecting at either end of the yolk, known as the chalazae (Fig. 

 3). Additional albumen, which has been secreted abundantly 

 in advance of the ovum by the glandular lining of the oviduct, 

 is caught in the chalazae and during the further descent of the 

 ovum is wrapped about it in concentric layers. These lamellae 

 of albumen may be easily demonstrated in an egg which has had 

 the albumen coagulated by boiling. The albumen secreting 

 region of the oviduct constitutes about one-half of its entire 

 length. 



The shell membranes which consist of sheets of matted 

 organic fibers are added farther along in the oviduct. The 

 shell is secreted as the egg is passing through the shell gland 

 portion of the oviduct. The entire passage of the ovum from 

 the time of its discharge from the ovary to the time when it is 

 ready for laying has been estimated to occupy about 22 hours. 

 If the completely formed egg reaches the cloacal end of the 

 oviduct during the middle of the day it is usually laid at once, 



