6So 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 striae vertical to its surfaces. 



The cells of this layer are exceedingly small columnar cells, 

 3 [j, 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 



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 [322], 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 [337] 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 



