BIGELOW: EARLY DEVELOPMENT OF LEPAS. 85 
Cirripedia which have ellipsoidal eggs surrounded by a rigid vitelline 
membrane. 
The rotation appears to be due to the mechanical relations existing 
between the dividing ovum and the vitelline membrane. 
« The first cleavage is a typical case of unequal cell division ; this is 
widely at variance with the account given by Groom (see the following 
review of the literature). 
3. REVIEW OF THE LITERATURE ON THE FIRST CLEAVAGE. 
According to the accounts or figures of Fillippi (65), Miinter und 
Buchholz (’69), Hoek (’76), Lang (78), Nassonow (’87), and Groom 
(’94), the first cleavage plane in all the species of Lepadidz and Balan- 
ide, which have been studied by them, is generally transverse to the 
chief axis; but it has been sometimes described as occasionally more or 
less oblique owing to variation. These investigators noticed that the 
long axis (chief axis) of the unsegmented ovum coincides with the long 
axis of the vitelline membrane, and that in the two-cell stage the plane 
of separation is transverse to that axis. These positions of the egg 
with reference to the vitelline membrane before and after cleavage led 
to the view that the first cleavage plane is formed at right angles to the 
chief axis of the egg, i. e., that cleavage is equatorial. Had the position 
of the polar cell during and after cleavage been carefully observed, this 
view would not have gained acceptance. Of the above named authors 
Groom and Nassonow have figured the polar cell in the two-cell stage, 
and they represent it as situated in the original position near the 
rounded end of the vitelline membrane, 90° from the cleavage plane. 
Nussbaum (’87, 790) observed in some ova of Pollicipes cleavage 
planes in various degrees of obliquity with reference to the vitelline 
membrane, from nearly longitudinal to transverse. He is the only 
author who has figured or described a polar cell as lying in the cleavage 
furrow of the two-cell stage of a cirripede egg. Nussbaum explained 
these varying positions of the cleavage plane and polar cell with refer- 
ence to the long axis of the vitelline membrane by assuming that the 
ovum divides almost longitudinally, and that after division the egg 
turns within the vitelline membrane. The various positions of the first 
cleavage plane, which were observed by Nussbaum in different eggs, 
were assumed to represent phases in the turning of the egg as it rotated 
from the position in which the forming cleavage plane is nearly longitu- 
dinal to the final position, in which it is transverse. Nussbaum sug- 
