240 ALICE THING 



Regaud and Dubreuil ('08, p. 152) deny any protoplasmic 

 connections between the egg and the cells of the egg epithelium. 



In a fully developed ovarian follicle (rabbit) the zona is formed of 

 three concentric layers. The first is a very thin internal layer applied 

 to the surface of the egg but substantially independent of it; to this layer, 

 which is not homogeneous but fenestrated in the manner of a grating 

 (or grill?), we have given the name fenestrated epiovular membrane. 

 The second is an external layer in connection with the prolongations 

 of the cells of the corna radiata : it is formed by a thick felt of filaments 

 running in all directions, the felted layer. The third or middle layer, 

 the zona pellucida properly called comprises two substances, radiating 

 filaments irregularly extending from the felted layer to the periovular 

 membrane and an amorphous or granular substance (following the 

 action of the fixative), laid down in abundance in the spaces between 

 the radiating filaments which it bathes. . . . The felted filaments, 

 the radiating filaments and the epiovular membrane which have been 

 interpreted up to the present time as anastomosing elements are not 

 protoplasmic but an exoplastic production of the follicular cells about 

 the egg. 



The investigations of Rubaschkin and Waldeyer have left 

 them in doubt as to the exact origin of the homogeneous sub- 

 stance (so-called by them) of the zona pellucida. According to 

 Waldeyer ('01, '02, '03) the zona pellucida is composed of a fiber 

 felt and a homogeneous substance across which protoplasmic 

 connections from the epithelium to the ooplasm make their way. 

 The homogeneous substance is perhaps a product of the ooplasm 

 and the mammalian zona pellucida, derived in part from the 

 epithelium, in part from the ooplasm. Rubaschkin ('05, p. 519) 

 describes as the zona pellucida in guinea pigs, a thick homogene- 

 ous layer directly surrounding the egg or yolk membrane. Cen- 

 tral processes from the epithelial cells penetrate this zona sub- 

 stance where they lose their protoplasmic appearance. These 

 processes do not form intercellular bridges because they are pre- 

 vented from actual contact with the ooplasm by the presence of 

 the egg membrane. They do not end with enlargements or knobs 

 as Retzius figured them to do. A number of eggs, however, show 

 a thick layer of coarse fibers, the processes of the epithelial cells 

 which wind about the zona substance but do not penetrate it at 

 any point. This layer corresponds to the perizonal fiber net of 

 Retzius. Waldeyer is inclined to regard the zona pellucida, con- 



