6 4 



FECUNDATION; MOTILE ISOGAMETES. 



Using improved methods Timberlake ('01) in a study of spore- 

 formation in Hydrodictyon utriculatum Roth., has found that, in the 

 earlier stages of the process, cleavage takes place by means of surface 

 constrictions of the plasma membrane on the outside and the vacuolar 

 membrane on the inside of the protoplasmic layer, as may be seen from 

 Klebs' figures (Fig. 18, B, C). The process is a progressive one, the 

 cleavage furrows cutting out first large irregular multinucleated masses 

 of protoplasm, which are in turn divided into smaller ones, until each 



B 



FlG. 18. Cell-cleavage in Hydrodictyon utriculatum. (After Klebs.) 



A, cell showing cleavage furrows at early stage in the process ; e, place in 



protoplasm free from chlorophyll. 



B, sausage-shaped protoplasts formed in early stage of cleavage. 



C, two protoplasts similar to those in B, showing manner of further cleavage. 



D, final result of the cleavage. 



contains a single nucleus. In this manner the entire 

 protoplast is divided into uninucleated spores or gam- 

 etes, as the case may be. 



Judging from Strasburger's account of the process 

 in Ulothrix, it seems probable that cell-formation 

 leading to the development of gametes or swarm-spores is also a 

 cleavage similar to that in Hydrodictyon. In Ulothrix, however, 

 the cells are uninucleate, and a nuclear division must either accompany 

 or precede cell-division. Until the behavior of the nucleus is known, 

 and the process carefully worked out with the aid of more improved 

 methods, the exact nature of the cell-formation in question must 

 remain largely a matter of conjecture. 



In the light of more recent investigations concerning cell-formation 

 among the lower thallophytes, it is evident that our present knowledge 

 of this process in connection with the development of gametes or 

 asexual zoospores among the algse is very meager and fragmentary. 



