ZONA PELLUCIDA TN TURTLE EGGS 249 



ular part of the zona pellucida (f.s.) already observed in the first 

 stage. Immediately beneath the epithelimii in the zona (s.) 

 a series of polygonal or circular fields occupies an area correspond- 

 ing to that originally marked off by the primary network. On 

 the whole one receives the impression that merging occurs be- 

 tween the primary and the secondary networks so that distinc- 

 tion between them is no longer possible. The three elements of 

 which the outer layer is composed also make up the clear inner 

 striated layer though in the latter region the network of the 

 fundamental substance of the zona stains far less deeply and 

 appears to be of a much less dense character. The striations 

 (fig. 7, f.s.) are undoubtedly produced by filaments connecting 

 the epithelial cells with the yolk and by walls of the tubes of the 

 fundamental substance of the zona which these filaments traverse. 

 Since tubes, canals and filaments occur in the outer layer it seems 

 at first remarkable that the striations in it are not obvious in 

 cross sections. In favorable and largely decolorized preparations, 

 the outer layer does appear striated but in more darkly stained 

 preparations the fundamental substance obscures the prolonga- 

 tions because of its great affinity for the stain. The striation 

 in the inner layer is quite evident in cross sections because its 

 fundamental substance takes up very little stain. The inner 

 layer is evidently the older part of the zona and must have been 

 originally identical in substance with the outer layer, the later 

 differentiation resulting from a change in properties of the older 

 fundamental substance causing it to become less dense and to 

 have le* affinity for stains. For the site of active proliferation 

 of the fundamental substance is the surface of the epithelial layer 

 which moves back as the epithelial cells withdraw in the cen- 

 trifugal growth of the egg. It is a still more significant fact, I 

 believe, that living eggs show striations in the outer layer also: 

 at least I have lately observed this appearance in preparations 

 of more advanced stages of the living eggs of Aromochelys odo- 

 ratus, the eggs of which differ in no essential manner from those 

 of the species previously mentioned in the general structure of 

 the zona pellucida. In eggs of A. odoratus approximately 1.5 to 

 2 mm. in diameter examined in normal saline the striations of the 



