SUDLER: DEVELOPMENT OF PENILIA. 
113 
nothing that I could call a polar-body. This structure is present in 
Daphnia similis until the eight-celled stage according to Lebedinsky 
and the sixteen-cell stage in Moina according to Grobben. These 
observers did not see the formation of the polar-body, but state that 
it is formed while the egg is in the ovary. From these descrip¬ 
tions it seems to me that there is a possibility of its not being a 
polar-body but a disintegrating nucleus of one of the ovarian cells 
which has entered into the composition of the egg. 
The Position of the Egg in the Brood-Chamber. 
The eggs when laid lie in the brood-chamber with their long axes 
parallel to that of the mother where not more than two are laid 
on either side. In large individuals the orientation may be the 
same with a larger number of eggs; but in most cases where the 
number on a side is more than two, the eggs overlap so that the pos¬ 
terior end of each egg is outside the anterior end of the egg directly 
behind it. In some cases this position is not assumed until cleavage 
has advanced to the four- or eight-cell stage, and in a few cases it 
may not be assumed until the appearance of the appendages. Later 
on the change in position is controlled by different factors, such as 
the change in the shape of the embryo and the change in the shape 
of the enlarging brood-chamber. These will be described later. 
The embryo is nourished by means of the blood-plasma or albu¬ 
men with which it is surrounded. Its increase in size is due 
entirely to material obtained in this way. In the preserved speci¬ 
mens this nourishing substance has coagulated and causes the 
embryos to adhere quite firmly to the mother. In sections showing 
a segmentation cavity we find a coagulum slightly stained which 
probably comes from the same source. 
Segmentation. 
The first cleavage plane is transverse to the long axis of the egg 
and divides it into two separate and distinct cells, the anterior of 
which is slightly larger than the posterior. (Fig. 7.) 
The next plane of division is through the long axis of the egg 
at right angles to the first and is almost parallel with a plane pass¬ 
ing through the middle line of the adult. A four-celled embryo 
is thus formed with the two anterior cells slightly larger than the 
two posterior ones. (Fig. 8.) 
