6 EDWIN G. CONKLIN 
trifugal force from taking this eccentric position the resulting 
cell division may be nearly equal, or a giant second polar body 
may be formed (fig. 11). 
2. Cleavage. The first cleavage of Crepidula and of Fulgur 
is approximately equal. The pronuclei lie near the animal pole 
of the egg, the egg nucleus lying somewhat nearer the polar 
bodies than the sperm nucleus. The first cleavage spindle is 
oriented so as to lie at right angles to the egg axis, but it is im- 
possible in these eggs to determine whether the spindle les in 
a particular cross axis or not. However in typical cases the 
spindle invariably lies at right angles to the chief axis with its 
equator in that axis, and the protoplasm and yolk are divided 
by the first cleavage plane with strict equality. The same is 
true of the second cleavage which in all these regards resembles 
the first. 
It has been generally assumed that equal cleavages, alternately 
at right angles, are due to simple mechanical conditions, such as 
the greatest diameter of the protoplasmic mass, and that they 
require no further explanation. As a matter of fact equal cleav- 
ages, and successively alternating ones, cannot be explained in 
so simple a manner. The fact that the first cleavage spindle 
invariably stands at right angles to the chief axis of the egg 
and with its equator in that axis shows that there is here some 
orienting power of the highest significance. It is well known 
that there is considerable variation in the path which the sper- 
matozoon takes through the egg, and in its manner of meeting 
with the egg nucleus; there is also-much variation in the actual 
positions of the cleavage centrosomes and in the initial position 
of the first cleavage spindle, without any corresponding variation 
in the final position of the spindle or of the cleavage plane. As 
a result of the study of large numbers of eggs of many different 
animals, under both normal and experimental conditions, its 
seems to me necessary to conclude that the same factor which 
brings about an unequal division of an egg such as that of Unio, 
operates to cause the equal division of an egg like that of Crepid- 
ula; this factor is to be found in the polarity and symmetry of 
the egg itself. In Unio, where the first cleavage is very unequal, 
