CYTOLOGICAL STUDIES OF CEXTRIFUGED EGGS 647 
O, P, Q, U) to various degrees, the defective side being that derived 
from the region of the microblastodisc. The details of structure 
of these embryos will be described later. The behavior of the 
blastodisc during cleavage I interpret to mean that the first sperm 
nucleus entering from the blastoderm brings about normal fer- 
tilization. Others entering later are carried to the edge of the 
blastodisc, and failing to enter, produce a small blastodisc at one 
side. The presence of this blastodisc interferes with the normal 
development of the blastoderm. In other cases many sperm 
entering at about the same time are carried into the blastoderm, 
and produce there the polyaster that gives the multiple cleavage. 
No embryo, or even a part of an embryo, forms under the latter 
conditions. 
I was not without hope that the microblastodisc might give 
rise to micro-embryos, paternal in origin, but although I have 
had hundreds of such blastodiscs none have produced embryos, 
^lost of them owe their origin to more than a single sperm, and 
hence are probably derived from polyastral figures. Under such 
circumstances normal development might not be expected judg- 
ing from the behavior of the large polyspermic blastoderm. But 
in some cases, where the microblastodisc divided at once into two, 
it is probable that a single sperm is responsible for its formation 
and cleavage, yet I have obtained no embryos from these micro- 
blastodiscs, when eggs containing them were isolated. I can only 
attribute this failure to the minute size of the mass; for, to judge 
from what happens in uninucleated pieces of the sea-urchin eggs, 
paternal embryos are possible formations. 
Thinking that the smallness of the blastodisc might be due 
to the paucit}^ of protoplasm in its vicinity I tried the effect of 
centrifuging no more than was enough to move the egg within its 
membrane, and not enough to carry the peripheral protoplasm 
into a disc. The results were the same as before, and the slight 
difference in size of the microblastoderm, if such differences 
exist, produced no effect, for the greater part of the protoplasm 
flowed into the large blastodisc. 
I was also not without hope that the sperm entering at a remote 
point from the artificial blastodisc might fail to reach the blasto- 
