504 Wisconsin Academy of Sciences^ Arts, and Letters. 



teresting when taken in connection witli the fact that in the 

 process of cleavage the same order of events is frequently ob- 

 served. 



As is kno^^Tl, anv cell of a net may foi-m spores vrithout the 

 accompanying cells being visibly affected. It generally hap- 

 pens, however, that a number of the cells are in some stage 

 of spore formation at the same time. I have frequently found 

 in my material single nets of which the majority of the cells 

 were in a vegetative condition while one might show stages of 

 cleavage, another ciliated spores, and in still another there 

 w^ould be a newlv formed net. The relation of these' different 

 cells to another so far as their position in the net is concerned, 

 I could not make out, since in imbedded and sectioned material 

 the general arrangement is difficult to determine. I have not 

 made any special observations as to the age of the nets whose 

 cells produce swarm spores, although such an investigation 

 would be of great value. Ivlebs has shown that a nmnber of 

 special conditions of nutrition, etc., will cause spore fonna- 

 tion, but the age of tlie cells thus experimented upon was not 

 accuratelv determined. 



The first indication of the approach of cleavage consists of 

 the disappearance of the pyrenoid and increase in thickness 

 of the protoplasm. This latter j)henomenon is very clearly 

 shown in Fig. 32, which shows sections of two adjacent cells, 

 in one of which cleavage has just begun, while in the other 

 there is no indication of it. In the account of the process of 

 starch formation in Hydrodictyon I described the disappear- 

 ance of the starch and pyrenoids as usually occurring before 

 cleavage begins, but showed that such a process is not necessa- 

 rily preliminary to cleavage since it sometimes happens that 

 some of the starch and the pyrenoids may persist through all 

 the stages even to the spores and young cells formed from them. 



Cleavage itself is, as Klebs pointed out, a progi'essive process ; 

 but it is accomplished entirely by means of two sets of surface 

 constrictions instead of, as Klebs thought, by means of intra- 

 plasmic vacuoles. In the first stages in the process short fur- 

 rows that have no apparent special orientation with reference 

 to one another or to the nuclei appear here and there through 



