28 FORMATION OF SPORES OF RHIZOPUS AND PHYCOMYCES. 



The writer has also studied the cutting out of the columella and the 

 spores in Piloholus crystaUinu.s and Sporodhiia grcoidls^ but, as his 

 investigations agree with Harper's account (1899), he has simply 

 given a fuller review of his work than he should otherwise have done, 

 and shall treat these two genera in his general discussion. 



GENERAL CONSIDERATIONS. 



From a consideration of the preceding pages, we find that the 

 processes by which the spores are formed in Rhizopus and PJii/eomyces 

 appear very different, and that both these forms differ from Piloholus 

 and Spoiytdinia^ which two are different from each other. In other 

 words, of the four genera of the Mucorinea? that have been most care- 

 fully studied no two are alike. 



In Phycomyces the spore-plasm is divided into spores by vacuoles 

 alone. ^' The furrows cutting outward from the columella cleft form 

 no exception to this statement, this cleft being simply a fused system 

 of vacuoles. In Plloholns both vacuoles and surface furrows take part 

 in the process. In RJiizopus and Sporodhiia the work is done entirely 

 b}' surface furrows and furrows from the columella cleft, the vacuoles 

 in the spore plasm playing no part in the process. Sporodi/iia, how- 

 ever, differs from all the other forms in having none of the denser 

 plasm included inside the columella, and ivoxw Piloholus and Phizopus 

 in having no surface furrow to assist the vacuoles in cutting out the 

 columella. PiJobolus differs also from the other three forms in that 

 the spores in this genus only are cut down to a uninucleated stage, fol- 

 lowed by an embryonic development consisting of nuclear and cell 

 division. 



There are some respects, however, in which all four genera agree. 

 In all cases the protoplasm is divided progressivel}', the nuclei during 

 the cleavage are in a resting state, and aU the protoplasm in the spo- 

 rangium outside the columella is included withfn the spore walls, the 

 substance between the spores not being protoplasm but a slimy mate- 

 rial excreted by it through osmotic membranes. 



Harper (1899) has pointed out that this is not a process of free ceil 

 formation in the sense that the cells are cut out entirel}' within a larger 

 mass of protoplasm, as in the ascus of LacJniea and Ki^ysipdie^ but is an 

 entirely different type of cell division. He uses this as evidence against 

 the homology of the sporangium of the ZA'gom^'cetes with the ascus. 

 Juel (1902), however, in a very, recent j)aper on Ttqyhridiinn (a new 

 genus of the Protomycetes) seems to have entirely missed this part of 

 Harper's distinction. He refers to the action of the kinoplasmic rays 

 as being Harper's whole distinguishing characteristic of free cell forma- 

 tion, and considers this insufficient grounds for such a distinction. He 



« By this is meant not that the vacuoles are the sole and active agents of division, 

 but that they are not assisted by surface furrows. See note to page 31. 



