STUDIES OF FERTILIZATION 419 
a. The spermatozoon is readily found in sections prior to pene- 
tration, and all parts can frequently be recognized, though the 
tail is usually invisible owing to extraction of the stain. The 
middle piece is always distinctly differentiated, but it is rather 
variable in appearance. Its ring-form is usually distinguishable 
in sections and in some cases One or more minute granules may be 
distinguished init. The head stains solid black in the iron haema- 
toxylin. Fifteen minutes after insemination (fig. 2, a, 6, c) the 
perforatorium has penetrated the vitelline membrane and is in 
contact with the fertilization cone. 
The fertilization cone itself presents a rather remarkable appear- 
ance, and it deserves very careful attention, owing to the role 
that it plays in the penetration of the spermatozo6n. Fifteen 
minutes after insemination it projects somewhat above the con- 
tour of the egg, and its substance, which is practically homogene- 
ous, stains much more deeply in the iron haematoxylin than the 
neighboring cytoplasm. This difference in staining reaction so 
suddenly acquired indicates a modification of its substance which 
might be conceived as due to slight coagulation by a fluid intro- 
duced by the spermatozoén. In ary event the change in stain 
is due to its relation to the spermatozo6n, and it is the first indi- 
eation of its conversion into a specialized cell-organ. 
A fertilization cone has been described in various eggs (echinids, 
asteroidea, ete.), but in all eases previously described its signifi- 
cance has been, apparently, merely temporary and local, a reac- 
tion of the ovum to the spermatozo6n with no definable function 
of its own. The case of Nereis, however, is very different, as will 
be seen from the following description. 
At twenty-seven minutes after insemination the fertilization 
cone no longer forms any projection (fig. 3); it is usually, on the 
contrary, somewhat depressed in the center. The perforatorium 
stains somewhat more strongly than before. 
At thirty-four minutes after insemination the perforatorium has 
entered the surface of the fertilization cone to a slight extent 
(fig. 4), and a small granule appears at its tip, surrounded by a 
slight clear area. <A little later (fig. 5, a, 6, c, d, thirty-seven 
minutes after insemination) there is a group of three or four black 
