MUSEUM OF COMPARATIVE ZOOLOGY. 37 



ingly difficult to decide whether the faintness of color is due to a specific 

 difference of substance, or is simply the result of the tenuity of the film 

 itself. In the former case, one would perhaps be justified in concluding 

 that there was an inter-prismatic substance which served the purpose of 

 a cement to hold the stalks together. The peculiar longitudinal band- 

 like structures noticed during the elongation of the stalks (Plate I. Fig. 9, 

 g, h, I) are possibly to be referred to the same substance. 1 But on the 

 second assumption these shreds of faintly stained substance could be 

 hardly more than the lacerated edges of the stalks themselves. I con- 

 sider the latter the more probable explanation. 



Owing to the spherical form of the egg, tangential sections are circular 

 in outline, and in a given section the centre represents the deepest part. 

 When the centre of such a section is occupied by the superficial part 

 of the zona radiata, the periphery is formed by a circular band of the 

 villous layer, the deeper portions of which are nearer the centre of the 

 section. 



A segment from that portion of the band which cuts through the 

 bases of the villi, their roots, and the superficial portion of the zona, is 

 shown in Plate III. Fig. 5. Proceeding from the outer (in the figure 

 upper) toward the central portion of the section, one observes that the 

 cross sections of the villi increase somewhat in size, that the stalks 

 which embrace cavities become more numerous, and that the outlines 

 of the stalks become more and more star-shaped, and then irregular, 

 and that finally they break up into detached spots, which a little farther 

 along become smaller and smaller until they cannot be distinguished in 

 size from the pore-canals. 



Since the sections of the membrane are successively increasing in 

 diameter, the deep face of each will pass through a broader portion of 

 the zona than the upper face will. If for the purpose of examination 

 the section be inverted, so that the deep face is uppermost, the relation 

 of parts can be much more easily and satisfactorily studied than if it be 

 viewed from the upper face only, because the zona offers less impediment 

 to vision than the thick-set columns of the villous layer. Attentive 

 focusing shows conclusively on such preparations that the rays or 

 branching roots of the prismatic columns lead each to a pore-canal, and 

 it becomes possible in many cases to note the exact number of pore- 



1 Dilute hydrochloric acid causes the distance hetween the heads of the villi to 

 increase. This would be readily explainable as the result of the swelling of an 

 inter-villous substance, could the existence of such a substance be satisfactorily 

 established. 



