38 BULLETIN OF THE 



canals in which a given stalk takes root. The substance of the roots 

 and of their rib-like extensions up the stalk appears to be more deeply 

 stained than that of the expanded foot of the stalk ; but this is perhaps 

 only an appearance due to the fact that they are considerably thicker 

 than the membranous portion which connects them. 



As the successive sections pass through deeper and deeper portions of 

 the zona radiata, the calibre of the pore-canals grows very gradually 

 finer, and those which are plugged with deeply stained villous roots 

 become less numerous, but otherwise there is no essential difference 

 in the appearance of the sections. The characteristic arrangement of 

 the pore-canals previously described is visible here, and may be made 

 out more easily than on the fresh egg-shell, provided the sections are 

 made perpendicular to the canals and are sufficiently thin. 



In radial sections from eggs that have been hardened and stained, the 

 zona is usually of a uniform faint tint (Plate III. Fig. 1), but often 

 there is a very gradual deepening in the intensity of the color in passing 

 from the outer to the inner boundary of the layer (Plate II. Fig. 1 ) . In 

 a few instances this deeper stain seems to extend toward the outer sur- 

 face of the zona in flame-like jets (Plate IV. Fig. 1). The outer boun- 

 dary of the zona, although appearing slightly irregular, owing to the 

 variable lengths of the root-like prolongations of the villous layer, is in 

 reality fairly even and sharply marked (Plate II. Figs. 1, 7, 8, Plate 

 III. Fig. 1, and Plate IV. Fig. 1). The inner boundary is still more 

 precisely defined, and appears as a fine continuous line, which sharply 

 separates the zona from the peripheral layer of the yolk. Nowhere is 

 there any evidence of a gradual transition from the yolk to the mem- 

 brane. Occasionally, when the section is not exactly perpendicular to 

 the inner surface of the zona, this boundary appears double, but careful 

 focusing in such cases always shows this to be an optical illusion. In 

 a few instances I have seen a similar appearance which was not thus 

 explainable. For a considerable distance a layer of nearly uniform 

 thickness appeared to intervene between the zona radiata and the yolk 

 (Plate II. Fig. 7). But the line which separated this from the rest of 

 the zona was never to be made out for more than a small portion of the 

 circumference of a section, for it either terminated abruptly, or, gradu- 

 ally approaching the inner boundary of the zona, became confluent with 

 it. Its inconstancy and its want of continuity are together sufficient to 

 show that the layer in question is not entitled to be considered a dis- 

 tinct membrane, nor even a differentiated portion of the zona radiata. 

 I may add, that I have never seen a section of this kind in which it was 



