64 FECUNDATION; MOTILE ISOGAMETES. 
Using improved methods Timberlake (’o1) 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 
Fig. 18.—Cell-cleavage in Hydrodictyon utriculatum.—(After Klebs.) 
‘| A, cell showing cleavage furrows at early stage in the process; ¢, 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 zoéspores among the alge is very meager and fragmentary. 
