248 ALICE THING 



these meshes and of the development of ciiticiilar bridges con- 

 necting the parts as has been shown to take place in the membrana 

 olfactoria (Van der Stricht) although certain figures do suggest 

 such an interpretation. A superficial and older portion of the 

 veil of the zona pellucida shows the beginnings of the adult 

 structure, regular small round spaces inclosing dark granules, the 

 cross sections of prolongations of the epithelial cells (figs 5, pr.). 



Stage 2. In more advanced phases of growth the nuclei of the 

 epithelial cells are crowded nearer to one another and lie closely 

 on the zona pellucida. A cross section (fig. 7) shows that the 

 zona pellucida has become thicker and is divided concentrically 

 into two layers, the outer of which {o.l.) is more or less homo- 

 geneous and very dark in the figure whereas the inner {i.l.) is 

 less opaque and distinctly striated in a direction perpendicular 

 to the surface of the egg. This layer is separated from the yolk 

 substance by a sharp boundary, the nature of which together with 

 the two layers of the zona must be investigated in tangential 

 sections. The real importance of the study of tangential sec- 

 tions is well demonstrated here for the extremely intricate struc- 

 ture of the zona pellucida, of which one could obtain no true con- 

 ception from cross sections, is revealed with remarkable clearness. 

 In many preparations, as portrayed in figure 8, o.l. the outer 

 denser layer appears separated into three concentric belts, a 

 middle clearer stratum {s' .) between two bordering darker thicker 

 strata (s. .§".). In other preparations stained either more deeply 

 or very slightly this concentric division into belts is not see*i. 

 A completely satisfactory explanation for this phenomenon 

 cannot be given. There is a possibility that it may be due to 

 accidental causes, for instance uneven penetration of the fixative 

 or other media used though its explanation is more probably 

 to be found in differences in constitution between the older and 

 the more recently formed parts of the zona pellucida. 



Far from being homogeneous the outer layer consists of clear 

 spaces, the cross sections of a system of canals (c.) within which 

 are seen filaments, the cytoplasmic prolongations {pr.) from epi- 

 thelial cells. The canals are separated by a meshwork much 

 thicker and larger than in earlier stages, representing the cutic- 



