GAMETOGENESIS OF ORNITHORHYNCHUS 481 



sharply marked off from one another by intercellular channels 

 filled with intercellular substance. The latter undergoes early 

 a change of constitution and becomes transformed at the level 

 of the surface of the cells into the special cement known as the 

 terminal bars. 



The zona pellucida is formed by two or three different 

 elements. It takes its origin as a veil-like formation consisting 

 of a mosaic of terminal bars and polygonal helds within which 

 may be recognized the small, pale areas, future canals of the 

 adult membrane separated by pale and dark filaments giving 

 origin to the future fundamental substance of the adult 

 membrane. The fundamental substance of the zona pellu- 

 cida is developed as a cuticular element, by the terminal 

 bars or primary network, that is by a definite special inter- 

 cellular cement possessing the property of extension over the 

 free surface of the epithelial cells and forming connexions there 

 with the delicate secondary network apparently produced 

 directly by the superficial cytoplasm of the epithelial cells. 



With regard to the origin of the zona pellucida in mammals, 

 I believe that there are three possible methods of development : 

 the zona might develop from the follicular epithelium, it might 

 develop from the egg-cytoplasm, or it might be developed under 

 the influence of, and from, both egg-cytoplasm and follicular 

 epithelium. 



The majority of present-day workers appear to believe that 

 the zona of Mammalia develops from the follicular epithelium 

 alone. This is the view of such well-known older observers as 

 Flemming, Eetzius, Fischer, Von Ebner, Bonnet, and Eubasch- 

 kin. But Van Beneden, Sobotta, Waldeyer, and Kolliker all 

 believe that the zona pellucida is secreted by the egg-cytoplasm. 

 In support of this view are such observations as that of Van 

 Beneden, who described in a bat the fact that while there may 

 be two eggs in such close contact that at one place the follicle 

 is interrupted, yet at this region the zona is properly developed. 

 This is not of very rare occurrence in ovaries of placentals, and 

 is certainly difficult to explain if one believes that the zona is 

 of purely follicular origin. 



