. THE EGG. 231 



is comparatively free from yolk-granules. The nuclear mem- 

 brane disappears ; the chromatin elements form a reticular net- 

 work, which then becomes distributed through the whole nuclear 

 substance in the form of very fine granules ; and finally, these 

 granules run together to form six chromatin rods. The further 

 changes have not been followed with certainty in the hen's egg ; 

 and neither the formation nor the extrusion of polar bodies has 

 as yet been seen. 



The egg, which is now ripe, is discharged from the ovary 

 by rupture of the capsule at its most prominent part. The egg 

 is received at once into the open mouth of the oviduct, which is 

 closely applied to the ovary at the time, and then begins its 

 passage down the oviduct to the exterior. 



As it travels along the first or thin-walled part of the oviduct, 

 the albumen, or ' white of the egg,' is poured out around it from 

 the walls of the oviduct. The albumen is deposited as a con- 

 tinuous sheet, which is wrapped spirally round the yolk, owing 

 to the egg being caused to rotate, in its downward passage, by 

 spirally arranged folds on the inner wall of the oviduct. 



This rotation of the egg causes the spiral twisting of the 

 cords of denser albumen, at the ends of the egg, which are 

 spoken of as the chalaza? (Fig. 97, we). 



On reaching the lower part of the oviduct, or ' uterus,' the 

 shell membrane, and finally the shell, are deposited on the outside 

 of the egg, which is then passed into the cloaca, and laid. 



The egg takes about three hours to travel along the thin- 

 walled part of the oviduct ; in the uterus it remains for a vari- 

 able time, estimated by different authorities as usually lasting 

 from twelve to eighteen hours. 



3. Fertilisation of the Egg. 



The details of the fertilisation of the hen's egg have not yet 

 been determined. The large size of the egg offers great dif- 

 ficulties to the investigation of minute changes in connection 

 with the nucleus, and these difficulties have not yet been sur- 

 mounted. 



All that is known with certainty is that fertilisation is 

 effected, either in the upper part of the oviduct, or possibly, as 

 stated by Coste, before the egg leaves the ovary ; so that during 

 the whole time of the passage down the oviduct development is 



