MUSEUM OF COMPARATIVE ZOOLOGY. 33 



but seldom reach it ; most of them are traceable only a short distance 

 from the outer surface. They owe their prominence to the fact that 

 they are filled with a highly refractive substance having the form of a 

 corkscrew. "When the villous layer has been torn from the zona, this 

 substance appears to terminate exteriorly in a ragged, broken end, which 

 in some instances is drawn out into a tapering appendage (Fig. 10, a). 

 There can be no doubt — according to evidence to be gained from 

 ovarian eggs — that this substance is continuous with that of the pris- 

 matic columns of the villous layer, of which they are in reality the roots. 

 The roughened appearance of the inner surface of the separated villous 

 layer is largely due both to the lacerated ends of these roots and to the 

 fact that many of them are wholly withdrawn from the pore-canals when 

 the two layers are torn asunder. This relationship of the layers also 

 explains why it is so difficult to separate them over even a limited area. 



Sections of stained eggs, both radial and tangential, give instructive 

 views of the egg membranes. In radial sections the difference between 

 the villous layer and the zona becomes at once apparent from the deeper 

 stain which the former takes on. The pore-canals are also usually more 

 distinct than in the fresh egg, although the effect of certain acid pre- 

 servative reagents (Perenyi's fluid, picrosulphuric mixture) is such as to 

 obscure the radial markings of the zona. In the villous layer a still 

 more striking contrast is produced between the heads and the stalks of 

 the villi, since the former almost invariably take a much deeper stain 

 than the latter. Especially is this true when stained in picrocarmine, 

 by which the heads are colored a deep carmine while the stalks and roots 

 remain unstained or take a greenish-yellow hue from the action of the 

 picric acid (compare Plate IV. Fig. 1). In borax carmine both portions 

 are usually stained, and almost invariably the head much deeper than the 

 rest of the villus ; but it has occasionally happened that the heads were 

 less deeply colored, and presented a slightly yellowish tint (Plate IX. 

 Fig. 2). I am unable to account for the difference, unless possibly a 

 prolonged decoloration in hydrochloric acid is the cause of the feeble 

 stain of the head ends. In all these stained radial sections it is to be 

 seen that the transition from the head to the stalk, although not marked 

 by a sharply defined line, is nevertheless abrupt. Owing to this, and 

 the fact that the stalks, as well as the heads, are of nearly uniform 

 lengths, radial sections of well stained specimens always exhibit the 

 stalks in the form of a broad band or zone, sharply marked upon both 

 edges, — more deeply stained than the zona radiata, but less deeply 

 than the narrower well defined band which is made up of the heads of 



vol. xix. — no. 1. 3 



