32 FORMATION OF SPORES OF RHIZOPUS AND PHYCOMYCES. 



part of the plasma-membrane withovit any visible change except in 

 form. The protoplasmic surface that abutted against the vacuole is 

 the same that is later in contact with the cell sap in the clefts. The 

 boundary of the vacuole has become directly the boundary of a part 

 of the cleft. We have good reason, therefore, to believe that the 

 vacuolar membrane is identical with, or at least very similar to, the 

 plasma-membrane, and may serve the same purpose if opportunity is 

 offered. This homology is further substantiated by the fact that the 

 columella wall is laid down in the dome-shaped vacuolar cleft by the 

 plasma-membranes, formed for the most part by the vacuolar 

 membranes, and, in the case of Fhycomyce^ and Piloholns, the walls 

 of most of the spores are formed by what was once a number of 

 vacuolar membranes. If, with Strasburger (1898), we regard the 

 plasma-membrane as kinoplasmic, we find here very strong reasons 

 for believing that the vacuolar membrane is of a kinoplasmic nature 



also. 



The vacuoles are, then, openings in the protoplasmic mass, less 

 resistant to the contraction of the cytoplasm, and from which clefts 

 may originate. In the higher plants and in the ascus of the Ascomy- 

 cetes we have the new plasma-membrane of the daughter cells formed 

 by the kinoplasmic libers. In most animal cells and in many of the 

 alga?, as Cladophora^ and in the formation of conidia in fungi, the 

 new plasma-membrane originates from the old by following the con- 

 striction furrow from the surface inward. In FJnjcomyces there are 

 neither spindle libers nor surface furrows present during spore forma- 

 tion, and the kinoplasm which forms the plasma-membranes for the 

 spores seems to be located entirely in the vacuolar membrane. 



The behavior of the vacuoles in the sporangia of FilaJjoJus, Sj^oro- 

 dinia., Rluzopus^ and Phycoinyces is of considerable interest in its 

 bearing on the question of whether or not the vacuole can be consid- 

 ered as a permanent organ of the cell. Though, as already suggested, 

 the vacuoles are probably not active agents in the division of the pro- 

 toplasm, yet there can be no doubt that they do have a part to play in 

 the process by offering places of slight resistance to the contractions 

 of the cytoclasm, and by supplying material for the formation of new 

 plasma-membranes around the spores and the columella. In the cut- 

 ting out of the columella it is evident that the vacuoles are arranged 

 in Their definite dome-shaped system for the distinct purpose of being 

 where they can best do their part in the process. In Phycoinyces the 

 early formation of the stainable substance in some vacuoles, while 

 others remain empty, and the fact that the former go to form plasma- 

 membranes for the spores and the columella, while the latter do not, 

 indicate that certain vacuoles are predestined from a very young 

 condition of the sporangium to take part in columella and spore 

 formation. 



