32 BULLETIN OF THE 



These roots serve to fasten the villous layer very firmly to the zona 

 radiata, in a manner to be explained in connection with the account of 

 that layer. 



The much finer radial markings of the zona radiata (z. r.) are entirely 

 different in character from those of the villous layer ; seen from either 

 surface with a moderately high power they appear as punctations, dark 

 or light according to the focusing (Figs. 3, 8, Plate I.), evenly scattered 

 over the surface, and yet so arranged as to give the whole area a very 

 characteristic appearance. Although rather evenly distributed, they are 

 arranged in groups or systems. One may trace over a considerable area 

 a series of dots placed at the intersections of a system of imaginary equi- 

 distant lines crossing each other at right angles ; near by may be other 

 series, in which the systems of imaginary lines cross at angles varying 

 widely from that of 90°; in still other series, the lines are arcs of circles ; 

 the circles may vary somewhat in size, but the arcs are never to be 

 traced for more than a few degrees. These different systems abut upon 

 each other in the most fortuitous manner, and the intervening spaces are 

 filled with dots so evenly arranged as not to interfere with a fairly uni- 

 form distribution over the whole surface (compare Plate III. Fig. 5). 

 Higher powers show that the punctations are circular in outline, of very 

 nearly equal diameters (0.5 //, at their outer ends), and placed at inter- 

 vals averaging about 1.5 /x. My notes of May 24 and 25, 1883, make 

 the intervals between the pore-canals, as determined by measurements 

 on the shell of an ovarian egg that had lain in glycerine over night, 

 only f jx, less than half the value given above ; but I believe that the 

 larger distance fairly represents the average condition. 



Thin tangential sections show by focusing that these markings are 

 due to minute canals (pore-canals), which are ordinarily hollow, or at 

 least contain a substance that is less refractive than the common homo- 

 geneous mass of the matrix which they traverse. I have not seen any 

 evidence of a differentiation in the optical properties of the walls of these 

 pore canals which would allow one to speak of them as tubules. 



Weak hydrochloric acid causes the zona to swell slightly, and ulti- 

 mately renders the pore-canals less conspicuous or entirely invisible. 

 There are certain of them which do not fade away, however, even after 

 treatment with acid, and which at length become the only visible struc- 

 tures in what otherwise appears as a homogeneous layer. (Fig. 10. 

 Compare also Figs. 4, 6.) These canals very generally have a spiral 

 course, and are noticeably broader at the outer surface of the zona than 

 elsewhere ; they taper gradually toward the inner surface of the layer, 



