100 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
two primary segments. This cavity persists from this stage until the 
completion of segmentation." He does not refer to its recurrent char- 
acter in the earlier stages of cleavage, nor to the elimination of its 
contents in whole or in part. 
Joyeux-Laffuie (82), in his work upon Oncidium, a marine form with 
pulmonate affinities, makes no reference to a cleavage cavity. 
From my own observations on Planorbis and Physa, I have no doubt 
that the recurrent segmentation cavity is found in these forms, as in 
Limax ; but it is not developed in so marked a degree. I wish in this 
connection to call attention to the fact that the enclosing capsules and 
albumen of these forms are less dense than those of Limax, and that 
they are deposited in the water. In Planorbis, which has somewhat 
more yolk than Physa, the cavity does not attain so great a size as 
in Physa. 
I shall not enter into an extended discussion, or a review of the litera- 
ture of the cleavage cavity in other groups of animals, especially of 
marine forms. I shall refer mainly to those forms which, by reason 
of their conditions of development, might be expected to throw light on 
the significance of the cleavage cavity. 
In Spongilla, likewise a fresh water animal, Maas (790) finds no trace 
of a cavity in the solid morula stage, though he admits that there is at 
the four-cell stage the intimation of one, which later entirely disappears. 
According to Brauer (’92) a cleavage cavity appears in Hydra at the 
eight-cell stage, but he makes no reference to a subsequent disappearance 
of the cavity. 
ROTIFERA. 
Zacharias (85) finds a cleavage cavity in the two-cell stage of Philodina 
roseola, He does not figure it in the later stages, but speaks of its 
general appearance in all the eggs whose early stages he had observed. 
Zelinka (91) does not figure any cleavage cavity in the development 
of Melicerta or Callidina. 
ANNULATA. 
I have found no reference to a recurrent cavity in the marine forms 
of this group. In forms with much yolk, as Nereis, there may be no 
cavity whatever (Wilson 93); but in forms whose division is nearly 
equal, as in Eupomatus, a cavity appears at an early stage and persists 
until gastrulation (Hatschek *86). 
Whitman (78) describes a cavity in Clepsine, which appears very 
