1897.] Hammar’s Ectoplasmie Layer. 1029 
across the cell it does not affect the ectosarc which thus re- 
mains continuous from one cell to the other, figure 2. When 
the four cells formed flatten out 
against one another, figure 3, the 
HASAN ectosare is continuous all over 
Sa nA -the egg, and not cut by the cleav- 
Ney age planes. That this is still a 
living ectosare seems to be shown 
by the waves that rise up in it, 
especially over the smaller cells 
where they may be very high 
and sharp. 
Se Similar waves were seen in the 
E one cell stages of Yoldia, and 
may be common enough in Lamellibranchs. Thus Professor 
Brooks, in£1880,‘ called attention to contraction waves in the 
egg of the oyster. His figures indicate a clear outer layer con- 
tinuous over all the cells in various stages of cleavage, and, 
apparently, continuous with the polar bodies which adhere 
together;and to this layer up to a late stage of cleavage, and in 
spite of the fact that a membrane is thrown off early in the cleavage. 
It would seem necessary to interpret this layer as a living, 
membrane-forming substance, though with the low powers 
used, the wavy appearance figured was interpreted as being a 
wrinkling produced by contractions “which travel rapidly 
toward the formative pole, near which they disappear” (p. 42). 
These wrinkles vanished in about fifteen seconds, but came 
again upon other cells later in cleavage. It seems probable 
that further study will show that such waves are largely due to 
contractions in the ectosarc itself, as they seem to be in the 
cases I have examined with higher powers. 
The connection between such ectosareal portions of the 
Lamellibranch egg and the ectoplasmic layer of the Echino- 
derm egg is indicated by the appearance seen in figure 4. 
This represents a surface view of the opening into the cleay- 
age cavity of a four-cell stage of the Echnius common at Ros- 
* Development of the American Oyster. Report of the Commissioners of Fish- 
eries of Maryland, 1880. 
