STUDIES OF FERTILIZATION 435 
urements. In the third place the volume of the sperm nuc\eus 
normally increases considerably for some time after entrance, and 
the difficulty of deciding whether comparable stages are involved 
is sometimes great. 
In spite of these difficulties, however, I have found sperm 
nuclei which must be interpreted as partial on the basis of their 
size alone, especially in later stages. 
Fortunately however, it is not necessary to rely on size differ- 
ences alone for in some of these stages, as in the case of the pene- 
tration stages just considered, a remnant of the sperm head may 
be found adhering to the membrane at the point of entrance, 
guaranteeing the partial nature of the sperm nucleus within. <A 
few of these stages may be considered first: 
Fig. 32 shows a case immediately continuing those described in 
Section 3. Here the rotation had already begun, as evidenced 
by the position of the cone, a very delicate connection still remain- 
ing with the external part of the sperm head impedes the rotation 
of the sperm nucleus within. Fig. 33 a and b shows a more ad- 
vanced case; in fig. 33 a the cone and sperm nucleus are shown 
almost half rotated. One would not be able to decide from 
the size that it was a partial sperm nucleus, but the next section, 
shown in 33 6 contains a considerable portion of the sperm head 
still connected with the middle piece which has remained without 
on the egg membrane. This is a very critical case, demonstrating 
that a partial sperm nucleus will rotate like a complete one. The 
portion in 33 6 is entirely disconnected from the nucleus shown in 
33 a; they are in the very act of separation. In fig. 35 we have a 
very fortunate section in which a completely rotated fertilization 
complex: cone, sperm nucleus and aster, is present, and a con- 
siderable portion of the same sperm head is found on the mem- 
brane outside the egg; the external portion is entirely comparable 
to the condition shown in figs. 19, 24 and 25 and there can be no 
question about its interpretation. Undoubtedly, the condition 
came from one essentially similar to that shown in fig. 25; it 1s, in 
fact, exactly what one would predict a later stage of thecondi- 
tion of fig. 25 to be, assuming a break to occur between the oe 
within and without the membrane. 
