09 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



portions of the egg through secretions from the walls of the oviduct. 

 Thus, the layer of albumen surrounding the yelk is first deposited in 

 the passage of the ovum through the second, tubular portion of the 

 oviduct, the chalazse (see Fig. 9), or twisted, denser portions of the 

 albumen, being due to the rotatory motion of the egg against the spiral 

 ridges of the oviduct. The shell-membrane is formed by the organiza- 

 tion of the most external layers of albumen, and the shell is formed in 

 the third portion of the oviduct, or the uterus. The walls of this 

 portion of the tube secrete a viscid fluid which surrounds the egg, and in 

 which inorganic particles are deposited. The egg remains in the uterus 

 for from twelve to eighteen hours, and is then expelled through the 

 cloaca, narrow end downward, by its muscular contractions. 



Fig. 11.— Surface View of the Early Stages of Segmentation in a 

 Fowl's Egg, after Coste. {Foster and Balfour.) 



1 represents the earliest stage. The first furrow, B, has hegun to make its appearance in the centre 

 of the germinal disk, whose periphery is marked by the line A. In 2 the first furrow is completed across 

 the disk, and a second similar furrow at nearly right angles to the first has appeared. The disk thus 

 becomes divided somewhat irregularly into quadrants by four (half) furrows. In a later stage, 3, the 

 meridian furrows, B, have increased in number, from four, as in B, to nine, and cross-furrows have also 

 made their appearance. The disk is thus cut up into small central, C. and larger, D, peripheral segments. 

 Several new cross-furrows are seen just beginning, as ex. gr., close to the end of the line of reference, D. 



About the time the shell is being formed, provided impregnation 

 has taken place, changes occur in the blastoderm, which, though analo- 

 gous to the process of segmentation already mentioned as taking place 

 in the. mammalian ovum, yet differs from it. The germinal vesicle first 

 disappears, and a furrow is then seen to run across the germinal disk, 

 dividing it into two halves. This furrow is then met by a second run- 

 ning at right angles to the first ; this is then crossed by another, and 

 division of the segments proceeds rapidly by furrows running in all 

 directions until the germinal mass is cut up into an immense number of 

 minute masses of protoplasm, smaller toward the centre than at the 

 periphery of the disk. 



The furrows thus formed are not merely superficial, but extend 

 through the entire thickness of the germinal disk: hence the germinal 

 disk is cut up into minute masses of protoplasm. In other words, a 



