508 Wisconsin Academy of Sciences, Arts, and Letters. 



upon their further groAvth from the surface inward^ so that 

 the new cells are entireh^ surrounded by the portions of the 

 tvvo original membranes. The cleavage in the tTvvo cases, 

 then, so far as the mechanics of the process is concerned seems 

 to belong to totally different categories. Still if the pre- 

 vious suggestion that the cleft formed by the splitting of the 

 cell pJate is comparable to a vacuole prove correct it would be- 

 come more nearly possible to find a similarity in the two pro- 

 cesseSj but even in that case it must be kept clear that the sub- 

 stance for the increase in extent of the vacuole in the one case 

 is furnished by tlie spindle fibres for whose formation the nu- 

 cleus is probably a metabolic center while in the other there is 

 not the slis^htest direct evidence of anv such connection be- 

 tween the nucleus and the growth of the membrane forming 

 the cleavage furrow. To be sure there may be, as Harper has 

 suggested for Syncliitriuni, a diffusion of kinoplasmic ma- 

 terial from the nucleus to the plasma membrane, but here, as 

 in Syncliitriuni direct evidence for such diffusion is entirely 

 wanting. On the whole the process of cleavage in Hydrodic- 

 tyon seems to correspond most closely to that of the fungus 

 sporanges in which there is a progressive and complete cleav- 

 age, as for example in the formation of the protospores in 

 Synchitrium decipiens and Filohohis where the cleavage con- 

 tinues to the ultimate formation of the imi nucleate cells. 



A point of much si_gnificance in connection with the cleav- 

 age in Hydrodictyon is the fact tliat the two membranes which 

 take part in the cleavage are entirely discontinuous, although 

 they lie parallel to each other with a relatively thin layer 

 of protoplasm between them. But the two cleavage furrows 

 produced by these independent membranes from opposite sides 

 regularly meet in the midst of the protoplasm. This would 

 seem to render necessary the assumption that the impulse for 

 division is seated in the protoplasm between the two mem- 

 branes rather than in the membranes themselves. Such an 

 assumption need not involve the conception that the begin- 

 ning of the process takes place in the protoplasm betv/een, but 

 merely that the stimulus to which the cleavage is a response 

 is an internal stimulus acting upon the two membranes at 



