1897.] Embryology. 243 
The membrane that is formed after entrance of a sperm is made in 
the sea-urchin by the following spinning phenomena. In normal eggs 
of Echinus clear, homogeneous, straight, smooth filaments flow out from 
the egg, very close together, and all attain the same length about the 
same time. These filaments then seem to fuse at the tips to make a 
ceiling-like film that grows thicker. The space between the filaments 
is then filled in—in some undetermined way—and thus the membrane 
is completed. 
In abnormal eggs the spinning is irregular and more in the form 
of tufts and brushes that are more readily observed. 
In the star-fish after such a membrane is formed the egg sends out a 
tuft of threads from the place where the polar bodies were formed and 
then ceases to spin; soon the general surface spins again and continues 
to do so during cleavage. These filaments branch and anatomose ; they 
may bend suddenly at the base and even bend over at right angles at 
some point in their course ; they may start from the surface of the egg 
at various angles even tangentially ; they may run out and attach them- 
selves to the egg membrane. 
When cleavage of the egg begins, the spinnings show peculiar activ- 
ity near the plane where the cells will separate. As the cleavage 
furrow is formed threads spin from one side to the other so that the two 
cells are connected by cross filaments as fast as they are separated by 
the plane of cleavage. The liquid between the cells is crossed by many | 
most delicate strands and skeins of filaments that connect the two cells. 
When the two cells subsequently approach and flatten against one- 
another the connecting strands shorten and thicken as if contracting to 
draw the cells together. When pressure is applied to such cells the 
Connecting threads appear as if resisting separation of the cells and as 
if more active in drawing them together. 
Later when more cells are formed the same phenomena are seen and 
when the cleavage cavity is present it is crossed by a network of inter- 
laced filaments spun out from the inner ends of the cells. These fila- 
ments connect adjacent cells and also the most remote cells of the 
blastula, 
When the blastula is ready to swim, the external spinnings cease 
fora while and then start again as numerous processes from the general 
pellicle all becoming very long and active as the well known “ cilia 
that propel the blastula through the water. 
In the gastrula stage both entoderm cells and ectoderm cells spin 
filaments that cross the blastocæle and connect all the cells. When the 
mesenchyme cells are formed they too spin many filaments that connect 
