KOFOID: DEVELOPMENT OF LIMAX. 91 



division are periods at which the osmotic processes reach a maximum, 

 and thus the cleavage cavity may grow rapidly at this time. My obser- 

 vations on living eggs show that the period immediately preceding di- 

 vision is that of the most rapid growth of the cavity. It is not an 

 uncommon thing to find in the later stages neighboring cells in a 

 mitotic state enclosing a lenticular space between them. These two 

 causes may result in producing in some cases, during the early stages of 

 cleavage, an apparent rhythm between the nuclear conditions and the 

 periods of expulsion. There is, however, much variation in these early 

 stages, and it is impossible to establish in them any such constant 

 correlation as Warneck has indicated. 



2. Literature. 



Amphineura. 



No mention is made of any cleavage cavity in the development of 

 Dondersia, as described by Pruvot ('90). Kowalevsky ('83) does not 

 discuss the subject in Chiton, but Metcalf ('93) describes the cleavage 

 cavity as already formed at the four-cell stage. No statement is made 

 about its subsequent disappearance. 



LiAMELLIISRANCHIATA. 



I. Marine Forms. 



Loven ('48) does not figure a segmentation cavity in either Modiolaria 

 or Cardium. Barrois ('79) makes no reference to any segmentation cavity 

 in Mytilus, though his Plate XII. Fig. 16, if it represents & section, shows 

 such a cavity. He distinctly states that the segmentation produces a 

 body considerably larger than the original ovum. He also notes in the 

 two-cell stage the appearance, in one instance, of lenticular refractive 

 bodies apparently identical with those figured by Bobretsky as found in 

 Nassa mutabilis. These bodies are adjacent to the furrow separating 

 the micromere and macromere of the two-cell stage, and may be due to 

 a highly refractive secretion accumulated in these regions. 



Brooks ('80 a ) describes in Ostrea Virginiana a transparent cavity sepa- 

 rating the ectoderm from the macromere in dead eggs at a stage when 

 the macromere is almost covered by the very large number of ectoderm 

 cells present. He does not regard this space as normal, since the macro- 

 mere seems in living eggs to be in contact with the outer layer, and 

 there is no indication of a segmentation cavity. It is only concerning a 

 later stage, when the macromere has divided into a number of entoderm 



