KOFOID: DEVELOPMENT OF LIMAX. 101 
early at the place where the first three planes of division cross one 
another. Its early and later history is not given. No mention is made 
of any obliteration or recurrence of the cavity. He suggests that “ the 
blastocel, whenever it appears, forms as a necessary result of the cleav- 
age process. . . . The explanation of the cleavage cavity depends upon 
the fact that the cells push each other apart in cleaving.” 
This explanation was doubtless suggested by the cleavage of Clepsine, 
where there is considerable difference in size between the micromeres 
and macromeres, and tho less mobile yolk-laden cells adapt themselves 
less readily to the changes in cleavage than the protoplasmic blastomeres 
of the egg of Limax. he difficulty of applying this mechanical explana- 
tion of the cleavage cavity to the phenomena observed in Limax will be 
patent to all. The blastomeres, in this form at least, are exceedingly 
plastic bodies, adapting themselves either to the presence or absence of 
a cavity, upon which profound changes in their form depend. Further- 
more, the * pushing apart” of the cells in cleavage is often the occasion 
of the obliteration of a cavity rather than its formation ; for the cavity, 
in the early stages at least, is frequently at its maximum just before 
cleavage, and is obliterated or reduced in size at its close. 
Wilson (89) finds in the four-cell stage of Lumbricus a cavity 
which he labels “segmentation cavity,” and of which ho says, it “ dis- 
appears afterwards and cannot be identified with the true blastocol," 
which is described for the thirteen-cell and later stages. The eggs of 
L. foetidus, for which this ephemeral cavity is figured, have tough cap- 
sules and thick albumen, similar to that of Limax. 
Vejdovsky (88—92) describes a distinct ce vity in the two-cell stage 
of Rhynchelmis, and refers to the occurrence of a similar cavity in later 
cleavage stages. It is evident, however, from his figures, that the cavity 
is not so prevalent as it is in Limax. A similar cavity occurs in the 
two-cell stage of Allolobophora. A very interesting phenomenon was 
also observed by him in the six-cell stage of Allurus tetraëder. In the 
cytoplasm of the micromeres of this stage, a number of large contorted 
canals appear, which resemble very much the canals found later in the lar- 
val excretory cells of the embryo. These canals are filled with a clear 
fluid and “ hängen offenbar zusammen.” The accumulation of fluid in the 
canals results in an increase in the volume of the micromeres, and an 
obscuring of both cell boundaries and nuclei, Finally, by a powerful con- 
traction the fluid contents are expelled and the micromeres assume their 
original form and size, the cell boundaries and nuclei again becoming dis- 
tinct. It is evident that the author regards these canals as intracellular 
