264 
Cleavage. 
The cleavage of L. fascicularis is total and almost equal. The 
egg of this species contains less yolk than the eggs of L. anati- 
fera, L. Hillii, Pollicipes polymerus, Balanus bala- 
noides etc. The granular yolk is quite uniformly distributed, not 
aggregated into spherules as in most Cirripedia. In consequence of 
the small amount and the distribution of the yolk, the cleavage is 
much more regular than that hitherto described for any Cirripede. 
The successive divisions are regular and normally produce stages 
of 2, 4, 8, 16 and 32 cells. There is no resemblance to the ordinary 
types of Arthropod cleavage. The similarity to the regular cleavage 
of other Metazoa is almost as marked as in the cleavage of Lucifer. 
With one exception, to be noted later, every nuclear division is asso- 
ciated with total cell division. 
The first segmentation plane is at right angles to the long axis 
of the egg, and divides the ovum into slightly unequal cells (Fig. 1). 
Sections show that separation is complete and that there is no central 
mass connecting the cells. The larger cell contains most of the yolk. 
The smaller cell is the first ectomere, and marks the future anterior 
end. The yolk cell contains ectoblast together with the mesentoblast. 
The succeeding stages show that the early observers were wrong in 
regarding the first ectomere as the only one separated from the yolk 
cell. As will be seen, the ectoblast is separated from the yolk cell 
(mesentoblast) in several successively formed ectomeres, designated in 
the figures as A, B, Cand D. The yolk cell evidently has the value 
of a macromere in the embryological sense, though in L. fascicularis 
the term would hardly be applicable in the etymological sense. Since 
completing my observations, I have received GROOM’S paper, and find 
that in a study of other species and genera he observed, that the 
ectoblast is separated from the yolk in several successive ectomeres. 
In many eggs the first furrow remains at right angles to the long 
axis of egg, but in the majority the plane becomes shifted toward the 
left (viewed with polar body above, and in the line of the cleavage 
plane) so that the furrow becomes oblique to the long axis of the 
egg (Figs. 2, 3). One polar body lies in. the first furrow. The polar 
axis thus defined corresponds to the future dorso-ventral, the polar 
body being apparently dorsal in position. 
The second cleavage plane is practically at right angles to the 
first. Owing to the retarding influence of the yolk, division of the 
ectomere A is always slightly earlier than that of the yolk cell (Fig. 2). 
This gives rise to the so called 3-celled stage, which evidently led the 
