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 

 blastocoel, 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 micromerea 

 and macromei-es, and the less mobile yolk-laden cells adapt themselves 

 less readily to the changes in cleavage than the protoplasmic blastomeres 

 of the egg of Limax. The 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 "sagmentation cavity," and of which he says, it "dis- 

 appears afterwards and cannot be identified with the true blastocoel," 

 which is described for the thirteen-cell and later stages. The eggs of 

 L. fotidus, for which this ephemeral cavity is figured, have tough cap- 

 sules and thick albumen, similar to that of Limax. 



Yejdovsky ('88-92) describes a distinct cavity 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 tetraeder. 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 " hangen 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 



