JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 11 
that point. The walls of the oviduct invest the egg closely, and gener- 
ally cannot be removed, so that a thin membrane, if present, could not 
be detected. In most cases where it was possible to remove the walls 
of the oviduct entirely, no membrane could be seen either in sections or 
whole preparations, the egg appearing to be naked. In a fow cases, 
however, in which cleavage had recently taken place and the cleavage 
furrows were marked, it could be observed that the smooth outline of 
the egg was preserved even above the cleavage furrow, either by means 
of a membrane continuing across the furrow, or, what seemed from the 
appearance of the preparations more probable, owing to the presence in 
the furrows of a fluid mass, perhaps exuded from the egg. Such a case 
is shown in Plate 1, Fig. 4. On other grounds, however, it seems pos- 
sible that an extremely delicate membrane is present. Lameere (790) 
states that in Asplanchna Sieboldii, which is likewise viviparous, it was 
possible to observe definitely a very delicate membrane surrounding the 
egg, especially at the time of the formation of the polar cell. 
This question of the presence or absence of an egg membrane is of 
importance from a mechanical standpoint, owing to its bearing upon the 
question as to what preserves the ellipsoidal form of the egg. The form 
is retained throughout all the early developmental processes; cleaving 
cells do not project above tho general surface of the egg, nor do the 
products of cleavage become spherical, touching at a few points only, as 
is common in the Mollusea and other groups. This retention of the 
ellipsoidal shape by the egg compels the cleaving cells to take various 
peculiar forms, which allow of a direct test of some of the theories of 
cell division above stated. It is also a most important factor in the 
process of gastrulation, so that it becomes of great interest to discover 
what it is that preserves this form. 
It is ovident that surface tension would tend to produce a spherical 
rather than an ellipsoidal form. Roux (95) has recently proved that 
blastomeres have a direct attraction for each other; but an equal 
attraction throughout the mass would produce a spherical form, and an 
unequal attraction, such as would produce a regularly ellipsoidal form, 
is very difficult to conceive of, especially as this attraction would have 
to vary regularly with the shifting of the contents of tho egg. A mem- 
brane of equal elasticity in all parts would likewise result in the produc- 
tion of a spherical form. The only direct mechanical factor that seems 
capable of explaining the continued ellipsoidal form is the presence of a 
non-elastic membrane of the exact size and shape of the egg. But 
during the later development the embryo enlarges and changes its form ; 
