100 BULLETIN: MUSEUM OF COMPAKATIVE 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 ('90) 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 



