GENERAL CONSIDERATIONS. 31 



by green-colored arrows. It" the pulling- ut the sides of the furrows 

 continues, as in Piloholu.s^ the furrows arc wide, but if it soon ceases, 

 as in SynchitriuiK, they are narrow. 



It is not improbable that in the last stages of cleavage, where the 

 spores are connected by only a slender neck, constriction like that 

 which cuts off conidia may ph\v a part in finishing the process. 



There is little evidence that the nuclei directly intluence the con- 

 traction. The direction of the contraction seems to be in general 

 toward the center of the protoplasmic masses that are to be the spores, 

 without regard to the distribution of the nuclei. The nuclei do, how- 

 ever, seem to determine to some extent just what protoplasm shall 

 constitute each individual spore; otherwise we might have spores 

 formed of enucleated pieces of protoplasm, and nonc^ such has ever 

 been ol)served in these forms. 



Viewing the eleavage from the l)asis of localized contraction of the 

 cytoplasm, we do not find such radical dili'erences in the processes 

 involved in PUoholus^ Sporodinta, JiJilzopus^ and Plujcomyces as 

 appeared at first sight. In Piloholns and Plnjcounjcts there are large 

 vacuoles in the spore-plasm, in the vicinity of which cytoplasmic con- 

 tractions take place in such a wav as to cause angles to cut outward 

 from the vacuoles, while in SporocUnla and Phizopus such is not the 

 case. On the other hand, there are no c^'toplasmic contractions on the 

 periphery of the sporangium in PJajconiyces as in the other three gen- 

 era. Otherwise these four genera exhil>it no essential differences in 

 the manner of formation of the columella and spores. The difference 

 is simply in the location of the cytoplasmic contractions. 



The explanation offered for the mechanics of the cleavage in the 

 si^orangia of the Mucorineae seems equally applicable to other cases of 

 surface cleavage, e. g., Synchitrium^ Fulkjo^ and some animal eggs. 

 To illustrate this extended application of the theory the writer has 

 made diagrams of Synelutriuiu^ Fuligo^ and the egg of the squid, indi- 

 cating, by means of arrows, as in PI. VI, figs. 28-31, the location, direc- 

 tion, and duration of the cytoplasmic contractions that would produce 

 such furrows as have been observed in these forms. PI. VI, fiors. 32 



7 o 



and 33, are based on Harper's (1899 and 1900) figures, and PL VI, fig. 

 31, on Watase-s (1890) figure. If this view of the mechanics of cleav- 

 age be the correct one, we must regard the vacuoles as passive rather 

 than active agents in cutting the protoplasm." They have, however, 

 a very definite and important mission to perform. In all four genera 

 under discussion they form the greater part of the plasma-membrane 

 for the columella and for the surface of the spore-plasm next to the 

 columella, and in PlloboJuii and Phycomyces they form the greater 

 part of the plasma-membrane for the spores. As I have alreadv 

 stated, this is done by the vacuolar membrane becoming directly a 



" See note at the bottom of page 28. 



