30 BULLETIN OF THE 



into pieces, it is readily seen to be composed of the two layers men- 

 tioned, which for the most part remain firmly united. Along the torn 

 edges, however, it often happens that the lines of rupture in the two 

 layers do not coincide, so that for a considerable area there is a separa- 

 tion of one layer from the other (Plate I. Fig. 5). Such regions are the 

 most satisfactory ones for the separate study of the two structures. 



Both layers are translucent; to the outer belongs the brownish tint 

 seen in surface views, while the inner is slightly opalescent. When seen 

 from the edge or in optical section, or, better still, when cut with a 

 razor into thin radial sections, both exhibit radial striations, which are 

 much closer to each other in the inner layer than in the outer (Plate I. 

 Figs. 4, 5, 11). 



Aside from certain exceptions which will be considered later, the fine ra- 

 diate markings of the inner layer, or zona, appear as nearly straight par- 

 allel lines, which are traceable through the whole thickness of the layer, 

 but which become gradually less prominent toward the deep surface. 



The markings of the outer or villous layer, on the contrary, are less 

 uniform ; they traverse the whole thickness of the outer layer, but are 

 most clearly defined near the periphery, their deeper portions being 

 more irregular and confused, and often exhibiting a tendency to a zig- 

 zag course. They indicate the boundaries of highly refractive prismatic 

 bodies, of which the layer is composed, and which seen endwise produce 

 the appearance of polygonal areas already alluded to. When viewed 

 from its deep surface (compare Plate I. Figs. 2, 7), the villous layer 

 has a somewhat ragged appearance ; it also exhibits polygonal areas, but 

 they are less regular and less clearly defined than those seen from the 

 external surface. When the egg membrane has been left for some time 

 in water, or, better still, in a mixture of water and glycerine to which a 

 trace of hydrochloric acid has been added, the prismatic elements which 

 compose the villous layer undergo a remarkable change, during which 

 the cause of the peculiar zigzag appearance of their boundaries is made 

 evident. After a time the free rounded ends of some of the prisms 

 appear to protrude above the neighboring ones (Plate I. Fig. 4), thus 

 giving the surface a less even contour than it had at first. On com- 

 paring the conditions before and after the application of acid, it at once 

 becomes apparent that the layer has increased in thickness. At its free 

 edges the prisms become more or less detached from each other, and it 

 then is possible to appreciate their real form. 



There are recognizable at least three distinct regions in each prismatic 

 villus, and each may be roughly compared to a stalk of grain, with its 



