92 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
cells, and the embryo is much flattened in the dorso-ventral direction, that 
he says a distinct, “segmentation cavity, or more properly a body cavity, 
is now clearly visible” between the ectoderm and the entoderm. Horst 
(82) says, “ It is not possible to demonstrate the existence of a true 
cleavage cavity in the oyster." 
Hatschek (80) finds that one of the peculiarities of the development 
of Teredo is “der gänzliche Mangel der Furchungshohle.” 
IL Fresh- Water Forms. 
Forel (68, p. 14) called attention to the fact that the * yolk " in Unio 
later becomes clear and transparent, but be failed to interpret this 
x 
appearance as a cavity. It remained for Flemming (75) to show in 
Anodonta the presence of a cleavage cavity as early as the two-cell stage, 
to establish its recurrent character, and to assert its equivaleney to the 
cleavage cavity of the later stages. He notes its formation as a lens- 
shaped cavity between the macromere and micromere of the two-cell 
stage ; its disappearance with approaching cleavage; its reappearance 
in the four-cell stage, and its subsequent disappearance before the next 
cleavage begins. He also notes its relatively large volume in a stage 
when there are from six to ten micromeres, but he does not describe 
any further obliterations of the cavity. The fact that the earlier cavities 
are obliterated does not raise the question with him as to whether they 
should be regarded as cleavage cavities or not. 
Rabl (76) has observed a similar phenomenon in Unio pictorum. He 
describes the cavity as a transparent protoplasmio layer with few yolk 
granules between the cells of the two-cell stage. He contends, however, 
that it is not the beginning of the cleavage cavity, as Flemming had 
maintained, and calls attention to the fact that similar regions, free 
from granular structures, between two or more cleavage spheres, are 
moet with in the embryos of other animals, as, for example, in many Gas- 
teropods, where the cleavage cavity appears later than it does in Unio. 
At the four-cell stage, however, he recognizes “die erste Anlage der 
Furchungshöhle,” but does not speak of any reduction or disappearance 
of this cavity in later stages, though some of his figures suggest it. 
Korschelt (91) speaks of an expansion of the primitively narrow seg- 
mentation cavity of Dreissena, during which the embryo, which has 
reached the gastrula stage, assumes a roundish oval shape. No mention 
is made of the ephemeral or recurrent, character of this primitive cavity. 
It is a matter of considerable interest to see that Dreissena, which is à 
“near ally of the common mussel,” and is probably a recent migrant 
