86 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
gested that the turn of the egg might be explained on the principle of 
least resistance, since the long axis of the divided egg can only be ad- 
justed to the long axis of the vitelline membrane. He failed to study 
sections of stages in the first division and to follow continuously the 
cleavage of a living ovum. Groom (’94) expressed doubt concerning, 
Nussbaum’s identification of the body in. the cleavage furrow as the 
polar cell, for it had not been followed continuously from its formation. 
Nussbaum’s figures of three different ova with cleavage planes respec- 
tively in almost longitudinal, in oblique, and in transverse positions do 
not give conclusive evidence in support of his assumption that the egg 
rotates after cleavage. Groom has remarked that, if a rotation occurs, 
an ovum with oblique cleavage plane should show a correspondingly 
situated polar cell, and Nussbaum’s figure of such a stage does not 
show this. So far as the evidence offered by Nussbaum is concerned, 
one might well accept Groom’s view, that the various positions of the 
first cleavage plane in different ova indicate merely variation of the posi- 
tion in which it forms. 
Although Nussbaum failed to support his assumption with conclusive 
evidence, he was certainly in the main correct, as the evidence offered in 
this paper proves. Studies of the preserved material have convinced me 
that the relations in Pollicipes agrees with those in Lepas. Nussbaum’s 
assumption that the rotation takes place after division does not agree 
with the facts in the case of Lepas. I have shown that the rotation 
takes place not after, but during division, and have suggested that the 
forces concerned in cleavage, reacting upon the rigid vitelline membrane, 
_are apparently the cause of the rotation of the dividing ovum. 
Groom’s account of the first cleavage is so involved with his descrip- 
tion of the separation of the protoplasm from the yolk during matura- 
tion that no sharp line is drawn by him between the two processes. I 
quote from his paper (’94, pp. 135-136) the following description : — 
“ The polar bodies become pale and disintegrated, and the external one often 
gets washed away. The protoplasm is at last mainly collected at the anterior 
pole of the egg, and the yolk at the other (Figs. 6,7). . . . The surface separ- 
ating the protoplasmic half from the yolk commonly intersects the ovum in a 
perfect circle, and marks off what will form the first blastomere. . . . Very gene- 
rally the line of separation of the protoplasm and yolk is almost accurately 
transverse, . . . I have frequently seen cases when the wall was accurately 
transverse, and the polar body situated apically (Figs. 6, 7). Lastly I have 
been able to watch the gradual formation of the protoplasmic half in a single 
ovum ; the line of junction in these cases was transverse from the first.” 
