MUSEUM OF COMPARATIVE ZOOLOGY. 39 



not possible to discover in some part of the layer evidences of pore- 

 canals continuous with those of the remaining portion of the zona 

 radiata. I am therefore convinced that the zona radiata is a single 

 homogeneous layer which is in direct contact with the surface of the yolk, 

 and is traversed by pore-canals which reach from the yolk to its outer 

 surface. 



When radial sections of the zona are broken, they occasionally show a 

 tendency to rupture in lines concentric with the surface of the egg, but 

 this is so rarely the case as hardly to be characteristic. The fracture is 

 usually irregular, and not dependent on any structural feature ; even 

 the pore-canals do not appear to have much influence on the direction 

 of the line of separation. The nature of these canals can be more readily 

 studied on sections of hardened specimens than on the fresh shell. Their 

 proximity to each other is not so readily determined from radial sections 

 as by means of the tangential sections already described. The same 

 general features which were mentioned in describing their appearance 

 on the fresh egg are usually visible with even greater clearness on those 

 which have been hardened. The distinctness of the pore-canals varies, 

 however, considerably in different specimens, depending undoubtedly 

 upon the refractive power of the mounting medium, which penetrates 

 the canals, as compared with that of the matrix of the zona itself. 

 Upon the most favorable preparations the canals can be easily traced 

 from end to end, so straight is their general course. At the periphery 

 of the zona they are uniformly somewhat broader than at its deep sur- 

 face ; but they taper so gradually as to make the difference in calibre, 

 even at their two ends, trifling. In the case of almost every canal a 

 slightly spiral course is noticeable near the outer end, whether it be 

 plugged with the root of a villus or not ; and throughout the whole 

 length there is usually the faintest trace of a wavy or zigzag course. 

 Aside from this, however, the canals are remarkably straight and paral- 

 lel. There are no enlargements or irregularities in the calibre, save 

 those which appear to result from the distention of the canal with the 

 substance of the villous roots already described. 



There still remain to be considered some peculiarities of the villous 

 layer, which either result from particular methods of treatment, or have 

 not been observed sufficiently often to allow one to consider them 

 characteristic features. 



Of those dyes which I have used, acetic acid carmine gives the sharp- 

 est differential staining for the heads of the villi. While the stalks 

 and roots remain comparatively pale, the heads (Plate II. Fig. 2) take a 



