492 
ZOOLOGY: E. G. CONKLIN 
in the polar body; and even when the two daughter cells are made 
equal by pressure or by centrifugal force the spermatozoon always 
lies in one daughter cell and not in the other. The same is true if the 
centrifugal force is appHed during the second maturation division; 
although the daughter cells may be equal in size the spermatozoon is 
found in only one of these cells. Subsequent events show that only 
that cell develops which contains the spermatozoon. 
In Crepidula and many other mollusks as well as in annehds and as- 
cidians the spermatozoon normally enters the egg at the beginning of 
the first maturation division and always before the first polar body is 
cut off. Indeed the first maturation spindle will usually remain in 
the metaphase, or middle stage of division, until a spermatozoon enters 
or until the egg is stimulated by other means (artificial parthenogenesis) 
to begin development. As soon, however, as a spermatozoon enters 
an egg the maturation division proceeds and the whole process of 
development is set in motion in the cell which contains the spermato- 
zoon. But the polar bodies, which do not contain the spermatozoon, 
never develop even though they may be as large as, or even larger than, 
the egg which does develop. 
The giant polar bodies of Crepidula resemble unfertilized eggs in 
that the cytoplasm remains diffused throughout the whole cell whereas 
in eggs after fertilization there is a fairly sharp separation of cytoplasm 
and yolk. Associated with this lack of segregation of cell substances 
there is also a lack of distinct polar differentiation in giant polar bodies. 
The intra-cellular movements which lead in the fertilized egg to the 
segregation of cytoplasm at the animal pole and of yolk at the vegetative 
pole do not take place in giant polar bodies. 
These giant polar bodies invariably contain a nucleus and they may 
contain samples of all the substances found in the egg; they may con- 
tain most of the protoplasm of the egg; they may be larger than the 
cell which does develop but the one thing which they lack is a sperma- 
tozoon, whereas the egg cell which does develop invariably contains a 
spermatozoon. We must conclude therefore that the giant polar 
bodies of Crepidula do not develop because they do not contain a sper- 
matozoon. 
The failure of normal polar bodies to be fertilized and to develop is 
generally held to be due to their small size, but even when these polar 
bodies are large as is sometimes the case in mollusks, polyclads and nema- 
todes they do not undergo fertilization and do not delevop though 
they sometimes divide once or twice. In one case only has the develop- 
ment of a polar body, or rather of both second oocytes, been observed. 
