336 
SAMUEL RITTENHOUSE 
ond or third segmentation. The polar bodies are not held by a 
membrane, as the egg is devoid of such a structure; neither are 
there any protoplasmic connections visible with a magnification 
of 212 diameters. Yet for a time they seem to be held near the 
egg by some means of attraction. The first polar body may 
segment once or twice. Usually about the time of the second 
cleavage the polar bodies either disintegrate or pass out into the 
water and are lost. 
FERTILIZATION 
Very little concerning fertilization could be made out on ac- 
count of the character of the egg. The ova and spermatozoa are 
discharged into the water and there fertilization takes place. It 
is impossible to follow the nuclear changes which take place dur- 
ing maturation, or the union of the male and female pronuclei 
in the living egg, because of the density of the cytoplasm, and 
because material could not be secured in sufficient abundance 
in the various phases for the preservation of the different stages 
for sections. There is no visible fertilization membrane given off 
after the penetration of the spermatozoa. 
CLEAVAGE 
Cleavage is total, equal and nearly regular, especially in the 
early stages. The divisions occur at short intervals, and the 
blastomeres soon move away from the center of the egg, thus form- 
ing a gradually enlarging segmentation cavity. The cells con- 
tinue to divide and arrange themselves into a single layer around 
the blastocoele to form a true blastula. The egg is not divided 
into an animal and a vegetative pole as the deuteroplasm and pro- 
toplasm are distributed evenly in all parts. But, as is customary, 
and for convenience of description I will call the part of the ovum 
from which the polar bodies are given off the upper pole, and the 
part of the egg opposite the lower pole. 
The first cleavage occurs a short time after the polar bodies 
are ejected. The plane of division is vertical ; the segmentation 
- furrow begins at the upper pole and gradually deepens until the 
