198 BULLETIN OF THE 



neither border of which is sharply marked. Between the two zones of 

 thickenings are stretched delicate nearly parallel threads, which I shall 

 designate as interzonal filaments:. 



The granular elements of the yolk are distributed with the same 

 uniformity as before, and are only so far modified in their arrange- 

 ment as might have been anticipated from the changes in form, posi- 

 tion, and extent of the archiamphiaster. The vitellus has again as- 

 sumed more nearly the circular outline, aside from the protuberance 

 caused by the archiamphiaster. That portion of the profile where this 

 conical protuberance joins the sphere presents a very slight reverse 

 curvature. In hardened eggs the contour of the low cone is constantly 

 distinguishable from that of the remaining vitellus by a fulness and 

 evenness which are quite as noticeable as in the preceding stages. This 

 is the more conspicuous in some specimens from a thickening in the 

 envelope of the vitellus at this pole. In extreme cases (Fig. 50) the 

 thickness of this structure (vm?) may reach 3.5 /x at the pole itself, 

 but it thins out rapidly, so that where the base of the conical elevation 

 joins the vitelline sphere it is quite indistinguishable. It is sharply 

 marked from the underlying stellate figure, and presents in glycerine a 

 clear, even, though not prominent, external outline. Its substance is 

 finely punctate, a thing which causes me to question the interpretation 

 (vitelline membrane) which I was at first inclined to give this structure. 

 In some cases, apparently in this stage of advancement, it is represented 

 by only a thin homogeneous cortical layer such as is depicted in the 

 preceding stage (Fig. 43).* 



The changes following upon the conditions last described lead directly 

 to the production of a small, more or less spherical body at the animal 



* I am in no way prepared to insist upon the identity of these two structures. 

 As regards the origin of that which is to he seen in Fig. 43, I have no hesitancy in re- 

 ferring it to a cortical portion of the yolk itself The structure exhibited in Fig. 50 

 presents peculiarities not easily harmonized with a like explanation of its origin. 

 Foremost is the fact of its low refracting power; secondly, its finely granular struc- 

 ture ; and, finally, its very unequal thickness within narrow superficial limits. 

 Whether it may not be a comparatively thin fluid exudation from the animal 

 pole of the yolk coagulated by the acid, or whether it may not owe its origin to the 

 albumen surrounding this region of the vitellus, are possibilities which 1 have not 

 been able definitely to accept or reject. If in any way dependent on the albumen for 

 its orifin, it is ditficult to understand what should induce it to take this very peculiar 

 form, and why it has so sharp and even an exterior. I can recall nothing in the 

 development of other animals with which it may possibly be compared, unless 

 perhaps with the exudation from the eggs of Batrachia, described by Bambeke and 

 0. Her twig. 



M 



