ZONA PELLUCIDA IN TURTLE EGGS 247 



then appear successively the clear cytoplasm and perhaps the 

 basal segments of the nuclei; next various segments through the 

 nuclei; and nearer the yolk, sections through the central spheres 

 and terminal bars and therefore through the incipient zona pel- 

 lucida. These tangential sections (figs. 3, 4, 5) prove that the 

 cuticle is composed of large polygonal fields (p./.) marked off from 

 one another by a system of dark lines, the terminal bars (Lb.). 

 These large polygonal fields are not homogeneous but inclose 

 smaller fields of similar outline formed by a fine pale network, 

 the meshes of which are a little thicker and darker at some points 

 and in close connection with the terminal bars, thus giving the 

 impression of extensions of the bars over the surface of the epi- 

 thelial cells. The meshes of this fine reticulum seem exactly to 

 overlie the deeper cytoplasmic network (fig. 4 c.n) of the cell 

 which arises from the interwoven filaments extending from the 

 central spheres. The zona pellucida then takes its origin as a 

 veil-like formation consisting of a mosaic of terminal bars and 

 polygonal fields 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. 



In older oocytes several changes take place. Those portions 

 of the network, in which the meshes are a little thicker and are 

 stained in the same way as the terminal bars, have become much 

 more numerous (figs. 5, 6). 



It may render the description clearer at this point to distin- 

 guish the network of darkly stained meshes which follows the 

 pattern of the original terminal bars around the large polygonal 

 fields, calling this the primary network ip.n.) from that which 

 follows the outlines of the original cytoplasmic reticulum, using 

 for this the term secondary network (sji.) Dr. Van der Stricht 

 observes a similar distinction in structures of the membrana 

 tectoria. The meshes of the primary network appear to send 

 out short extensions to the secondary network and to soften 

 their sharp angles so that these assume circular or oval shapes 

 rather than clear cut polygons (figs. 5, 6, p.n.) . So far I have been 

 unable to assure myself definitely of a longitudinal splitting of 



