196 EMBRYOLOGY 
layer. Upon the top of the funnel-shaped disk of white yolk lies 
the germinal vesicle, which, like the white yolk, consists of numerous 
protoplasmatic spherules; part of the contents of this vesicle 
shrivels up, and causes the vesicle to assume the shape of a disk, 
the “germinal disk.” The rest, the greater portion of the ovum, so 
far as it is surrounded by the vitelline membrane, consists of yellow 
yolk, composed of numerous granular globules of albuminous and 
fatty matter. 
The Ovum in the Oviduct.—The ovum, while still in the upper 
portion of the oviduct, is surrounded by the spermatozoa which 
have worked themselves through the oviduct from the cloaca. 
They swarm round the surface of the vitelline membrane, and one 
or more spermatozoa find their way into the germinal vesicle, and 
fuse with the contents of the latter. Upon this impregnation follows, 
DIAGRAMMATIC SECTION oF A FERTILIZED Ecc. 
A, Air-chamber at the blunt pole; Bl. Blastoderm; Ch. Chalaze ; S. Shell; 
S.M. Shell membrane; Vm, Vitelline membrane. 
while the ovum is still within the oviduct, the remarkable process 
of “segmentation.” This process consists of the division of the 
germinal disk by successive cleavages into a number of cells, which 
step by step build up the complex mechanism of the embryo. This 
segmentation being restricted to the germinal disk is called ‘“ mero- 
blastic,” in opposition to “holoblastic” segmentation, where, as in 
the ova of the higher Mammals, the whole material of the egg 
becomes segmented. 
The egg, having been received into the oviduct, is propelled in 
a spiral course by the peristaltic contractions of its walls, and 
receives from the glands of its lining membrane an accessory mantle 
of albumen, or the “white” of the egg. The average composition 
of this albumen is 12 °% of proteid matter, 1:5 fat, 0°5 saline 
matter, and 86 * water. The albumen is rapidly added, and 
owing to alternating denser and more watery layers, has a spiral 
