418 FRANK R. LILLIE 
narrows around the entire egg, the depression in the vitelline mem- 
brane therefore disappears and the spermatozo6n again becomes 
prominent externally. It remains external until about forty to 
fifty minutes from the time of insemination and then disappears 
rather abruptly within the egg. Its penetration coincides with 
the late anaphase or telophase of the first maturation division, or 
with the extrusion of the first polar body. 
A definite granule to which the tail is attached always remains 
on the membrane at the point of penetration. The cytological 
study shows this to be the middle piece, which does not enter the 
ege. Not only have I repeated this observation frequently in 
two successive seasons, but it has been demonstrated to classes of 
students at Woods Hole and to some of the investigators there. 
The middle pieceand tail of the spermatozoon do notenter in the ferti- 
lization of the egg of Nereis. In view of the emphasis which has 
recently been put on cytoplasmic contributions by the spermato- 
zoon in fertilization, I paid particular attention to this point in 
the summer of 1911, and was able to determine not only that it 
fails to enter at the same time as the sperm-head, but that it can 
be demonstrated external to the membrane up to the time of the 
first cleavage at least (cf. also Just, ’12). 
I have seen the penetration of the spermatozo6n take place in 
all positions from the animal to the vegetative pole, and I cannot 
say that there is any preferred point of entrance with reference to 
the poles. The existence of polyspermy, which is not infrequent, 
proves that there is no preferred meridian for penetration. So it 
would appear that any point on the surface of the ovum may be 
used for penetration. 
4. The phenomena of fertilization as seen in sections 
a Before penetration. Technique: The eggs are fixed for an 
hour in Meves’ modification of Flemming’s fluid made as follows: 
Chromic acid 0.5 per cent, 15 ec.; osmic acid, 2 per cent, 3.5 
ec.; glacial acetic acid, three drops. Staining in iron haematoxy- 
lin alone. 
