BIGELOW: EARLY DEVELOPMENT OF LEPAS. 79 
In those eggs in which it is oblique at the close of the first cleavage, 
the vitelline membrane appears relatively broader, and the divided 
ovum is easily adjusted to an oblique position within the membrane. 
Fifteen or twenty minutes usually elapse between the first external 
appearances of division and the complete separation of the cells. From. 
the cases which I followed continuously it appears that the cleavage 
begins within two to three hours after the formation of the second polar 
cell. 
During this cleavage the ova are seen to undergo a series of marked 
contractions, as shown in Figures 11 and 14. Immediately following 
each contraction the cleavage furrow deepens and the ovum rotates 
through several degrees. These phenomena are probably due to the 
action of the astral fibres, which, as will be shown later, are a well- 
marked feature of the cleaving ovum. The external appearances would 
lead one to think that the internal contractions occur spasmodically 
rather than continuously. Similar appearances were many times noted 
also in the later cleavages. 
Additional evidence in support of this observation concerning rotation 
of the dividing egg has been obtained from living eggs of L. fascicularis 
and a species of Balanus. In L. fascicularis (Plate 11, Figs. 95-97) the 
first polar cell has been observed to remain attached to the vitelline 
membrane at its blunter pole until after the close of the first cleavage, 
when the second polar cell, attached to the egg, has moved 90° from the 
blunt pole of the vitelline membrane. This observation is conclusive 
confirmation of my earlier observations on L. anatifera. 
While no observations have as yet been made on the living ova of 
species of Cirripedia other than those already mentioned, the study of 
preserved material of other species indicates that in these the first cleav- 
age takes place as in L. anatifera and in L, fascicularis. In L. hillii, L. 
pectinata, Pollicipes, and Balanus the chief axis coincides with the long 
axis of the unsegmented ovum and of the vitelline membrane. After the 
first cleavage, I find the polar cell in the cleavage furrow, which approx- 
imately coincides with a transverse plane of the vitelline membrane. 
So far as known similar relations exist between the ovum and the 
vitelline membrane before and after cleavage in the ova of all Eucirri- 
pedia ; therefore, it is very probable that cleavage takes place in the 
entire group as in L. anatifera. Van Beneden’s (’70) figures of Saccu- 
lina suggest that the same may also be true for the ova of Rhizocephalan 
Cirripedia. 
The internal phenomena connected with the cleavage could not be 
