246 ALICE THING 



Zona pelhicida 



Solely for purposes of clearness the developmental history of 

 the zona pellucida may be presented in three successive stages. 



The first stage covers those phases of formation in which the 

 zona pellucida, on cross section, is but a thin one layered cuticle 

 while on oblique and tangential sections the beginnings of a 

 reticular network are found. 



The second stage includes that period during which the zona 

 pellucida becomes divided into two concentric layers, the inner 

 thin and radially striated, the outer, denser with striations more 

 or less obscured. 



The third stage is co-extensive with the period of growth during 

 which both layers just mentioned become very much thicker. 



Stage 1. The terminal bars, as viewed on a cross section, divide 

 the epithelial cells from the oocyte by an apparently continuous 

 line which at first is thin and uniform but later becomes thicker 

 until it is a cuticle of double contour and of rather uneven outline 

 especially on its deep surface where it lies in connection with the 

 epithelial cells. On this front the junctions of the intercellular 

 substance, separating the lateral surfaces of the epithelial cells, 

 make with the bars triangular thickenings. The change in the 

 terminal bars initiates the development of the zona pellucida. 

 From the time when the cuticle reaches an average thickness of 

 1m it may be termed the zona pellucida (fig. 2, i.z.p.). Filaments 

 of the cytoplasmic network extending from the attraction spheres 

 {a.s.) seem to attach themselves directly to the deeper limit of 

 this cuticle (fig. 2) the actual structure of which is not demon- 

 strable on cross sections. Oblique and tangential sections, how- 

 ever, make it clear that the zona pellucida is of complicated 

 organization even at this early stage. It is perhaps well to ex- 

 plain at once that in an oblique or tangential section of an egg one 

 may see two, three or more irregular rows of epithelial cells, the 

 number depending upon the size of the egg and therefore upon 

 the curve of the epithelial layer. These represent cross sections 

 of the epithelial cells at various heights. These portions in the 

 section furthest away from the yolk show the bases of the cells; 



