148 BULLETIN OF THE 



maintained, not to be considered as vitelline membranes, but as the 

 superficial part of the protoplasm of the egg. 



His ('73) found that the cortical layer of the yolk in many ovarian 

 e^s is more finely granular than the rest of the yolk, and that it is 

 radially striate. This outer portion of the yolk he called the zonoid 

 layer. Many others have seen similar structures. According to the 

 accounts of some authors the zonoid layer is found only in eggs which 

 are not mature, and even then it is not always present. 



The condition of the egg membranes in Amiurus and in certain stages 

 of Esox has suggested the idea that similar appearances may in some 

 cases have given rise to a belief in the existence of a zonoid layer when 

 there really was none. A partial withdrawal of the egg protoplasm 

 occupying the pore-canals produces an appearance which at first sight 

 suggests the presence of a striate membrane internal to the zona ; in 

 fact, I at first supposed it to be a distinct membrane, and was the more 

 easily misled because in some cases it seems to be of nearly uniform 

 thickness. However, more careful study showed that it was not a mem- 

 brane, and that the appearance was due to fine threads of highly refrac- 

 tive substance stretching across a space between the inner surface of the 

 zona and the yolk. There are two things especially which make it im- 

 possible for me to believe that this is a normal condition : the great 

 variability in the thickness of the supposed membrane in different parts 

 of the same egg, and the fact that the radial striations are due to a sub- 

 stance which is more highly refractive than the substance, if any, filling 

 the intervening spaces. If, on applying reagents, there is great contrac- 

 tion of the yolk, either it is torn from the protoplasm in the pore-canals, 

 or the protoplasm contained in the pore-canals is suddenly withdrawn 

 from them and distorted ; in either case, there would be no appearance 

 of a zonoid layer. If, however, the protoplasm should not be withdrawn 

 from all the pores, but should in the case of many remain stretched 

 across the space between the zona and the yolk, as might no doubt fre- 

 quently happen, we should find the supposed zonoid layer more coarsely 

 striate than the zona, a condition described by recent authors. Such 

 an origin of the zonoid layer might also explain its absence in ripe eggs. 

 After the egg has attained its full size, the connection of the yolk sub- 

 stance with the canals would naturally be less intimate than at an earlier 

 stage, and then a contraction of the yolk would not be accompanied by 

 the stretching of any filaments across the space thus produced. 



Scharff ('87 and '87 a ) has recently described, within the zona radiata 

 in young eggs of Trigla, a zonoid layer, which subsequently disappears. 



