680 THE GENERATIVE ORGANS, 
Structure of the Chorion—In the ripe ovarian ova the yelk is 
seen to be enclosed in a double layer of cells and a structureless 
membrane. The latter is next the yelk, and is known as the 
vitelline membrane, the two layers of cells form the chorion. 
In some of the ovarian eggs both layers of cells are very distinct ; 
in others the cellular character of the inner layer is lost, and 
it appears to be converted into a cuticular membrane, with 
distinct but fine strize vertical to its surfaces. 
The cells of this layer are exceedingly small columnar cells, 
3 in diameter. The cells of the outer layer are larger, and 
correspond with the hexagonal fields seen in surface views. 
They are thickest and most easily demonstrated at the anterior 
egg-pole. 
The question whether the chorion is a cuticular or cellular 
membrane has been the subject of much controversy. The 
earlier writers all regarded the chorion as cellular, but the 
tendency in modern times has been to follow Leydig [822], who 
regarded it as a cuticular membrane. 
The young ova in the ovary are indubitably surrounded by 
a single layer of columnar epithelial cells, and I have traced 
these cells in numerous sections to their final condition, in 
which they form a continuous striated layer, when the nuclei 
of the original cells are still readily seen in sections stained 
with carmine. Other eggs in the same ovaries no longer 
exhibit these nuclei. : 
The second layer of cells, that corresponding with the 
hexagonal fields, appears later, and is only seen in ova ap- 
proaching maturity. I have been unable to trace the origin of 
these cells with certainty, but they appear between the deeper 
epithelial layer and the tunica propria of the egg-tube. 
Korschelt [887] has figured numerous young ova in which the 
epithelial layer exhibits a double layer of nuclei, so that it is 
probable that the outer layer of epithelial cells is developed 
from the inner layer. 
These cells form the outer layer of the chorion, and not, as 
Korschelt supposes, its whole thickness. Whether they are 
directly transformed into the cuticular tissue of this layer, or 
