68 ANIMAL PHYSIOLOGY. 
protection. The appearance of relative opacity in some of the 
parts marked off as above is to be explained by thickening in 
Fic. 71.—Female generative organs of 
the fowl (after Dalton). A, ovary; 
B, Graafian follicle, from which the 
cee has just been discharged ; C, 
yelk, entering upper extremity of 
oviduct ; D, £, second portion of 
oviduct, in which the chalaziferous 
membrane, chalaze, and albumen 
are formed; F, third portion, in 
which the fibrous shéll membranes 
are produced ; G, fourth portion laid 
open, showing the egg ead 
formed with its calcareous shell ; H, 
canal through which the egg is ex- 
pelled. 
the cell-layers of which they are 
composed. 
The Origin of the Fowl’s Egg.— 
The ovary of a young but mature 
hen consists of a mass of connect- 
ive tissue (stroma), abundantly. 
supplied with blood-vessels, from 
which hang the capsules which 
contain the ova in all stages of 
development, so that the whole 
suggests, but for the color, a bunch 
of grapes in an early stage. The 
ovum at first, in this case as in all 
others, a single cell, becomes com- 
plex by addition of other cells (dis- 
cus proligerus, etc.), which go to 
make up the yelk. All the other 
parts of the hen’s egg are additions 
made to it, as explained before, in 
its passage down the oviduct. The 
original ovum remains as the blas- 
toderm, the segmentation of which 
may now be described briefly, its 
character being obvious from an 
examination of Fig. 72, which rep- 
resents a surface view of the seg- 
menting fertilized ovum (odsperm). 
A segmentation cavity appears 
early, and is bounded above by a 
single layer of epiblast cells and 
below by a single layer of primi- 
tive hypoblast cells, which latter 
is soon composed of several layers, 
while the segmentation cavity dis- 
appears. 
The blastoderm of an unincu- 
bated but fertilized egg consists 
of a layer of epiblastic cells, and 
beneath this a mass of rounded 
cells, arranged irregularly and ly- 
ing loosely in the yelk, constitut- 
