34 BULLETIN OF THE 



the villi. Each of the heads has its external free surface more or less 

 rounded and not quite smooth, its sides nearly parallel and straight, 

 and its ill-defined deep face also tolerably straight. Along the last it 

 is distinguishable from the stalk, with which it is continuous, by its 

 greater refractive power as well as deeper color, and by a slight dimi- 

 nution in the size of the stalk. The last distinction becomes more 

 conspicuous the more the stalk is elongated. The differences between 

 head and stalk are emphasized by the fact that the villi have a greater 

 tendency to rupture along this line of union than elsewhere (Plate II. 

 Fig. 1, and Plate III. Fig. 1). The outlines of the free end and the sides 

 of the head are sharp, and in thin sections, especially such as cut the 

 heads crosswise, the margins seem to be limited by a narrow double- 

 bordered dark band (compare Plate III. Fig. 3), as though the head were 

 invested in a thick deeply staining membrane. Since I have never been 

 able to find evidence of the separation of any membranous structure 

 from the surface of the head, I am disposed to believe that the appear- 

 ance simply results from a differentiation of the cortical portion of the 

 head, which otherwise appears perfectly homogeneous. In some cases 

 this cap-like cortical part seems to exert a restraining influence on the 

 swelling of the central portion ; at least I interpret in that sense certain 

 conditions of the heads frequently met with. In such cases their sides 

 are not strictly parallel, especially when the villi stand in an isolated 

 position (Plate II. Fig. 1). The head, instead of being marked off from 

 the stalk by a constriction or shoulder of the ordinary form, has its 

 outline gradually broadened or flaring as it approaches the peripheral 

 end of the stalk, and its cap-like sheath appears to end abruptly with 

 edges which are slightly everted ; the connecting portion of the stalk is 

 as broad as, or even broader than, the basal end of the head, so that the 

 direction of the resulting shoulder is just the reverse of that commonly 

 seen. The most natural explanation of this appearance which occurs to 

 me is, that the free edge of the cap-like sheath is distended, and even 

 sometimes everted, by the swelling which takes place in the region 

 where head and stalk are continuous, and that the sheath in all probabil- 

 ity acts as a restraining investment in preventing any great distention 

 in the rest of the head. 



The stalks, in radial sections of eggs which have been subjected for 

 some time to the action of water before hardening, have the appearance 

 of comparatively slender columns, which are often slightly sinuous, but 

 in general nearly parallel. They taper at first quite rapidly for a short 

 distance from the head, and then only very gradually toward the basal 



