ULOTHRIX AND HYDRODICTYON. 63 



theory of a reduction division of the chromosomes, which at the time 

 received a wider acceptance than at present. 



In Hydrodictyon Klebs ('91) affirms that the process of cell-forma- 

 tion, giving rise to gametes or asexual swarm-spoi'es, occupies an 

 intermediate position between simultaneous and successive cell-division. 

 From what follows it will be seen that the process is a cleavage 

 similar to that occurring in certain Phycomycetes, but, using the 

 methods that he did, Klebs failed to perceive the true nature of the 

 process. His account in substance is as follows : 



The first indication of cleavage is manifested in the appearance of 

 numerous small clefts, pointed at the ends, in the plasma layer con- 

 taining the chlorophyll (Fig. 18, A). This can be seen in material 

 cultivated in darkness in a maltose solution, especially after the appli- 

 cation of a weak plasmolysing agent. These clefts soon become 

 longer and more numerous, neighboring ones thereby uniting with 

 each other, so that finally the entire chlorophyll-bearing layer is seg- 

 mented into pieces which are still connected, however, by fine plasmic 

 threads. The cleavage is not confined solely to the chlorophyll- 

 bearing layer, but extends into the colorless plasma in which the nuclei 

 are situated. The plasma membrane and the wall of the vacuole are, 

 on the contrary, unaffected. Previously to and during the cleavage 

 the plasmic layer concerned frequently undergoes a contraction, thus 

 giving rise to colorless spaces, so that this layer appears as a coarse 

 net, as Pringsheim ('71) has described for Bryopsis. These spaces 

 contain also some plasma, and, as the plasma membrane and wall of 

 the vacuole are continuous, the entire cell contents form still a unit, 

 as shown by plasmolysis. The continuation of the cleavage results in 

 the segmentation of the plasmic contents into numerous bands with 

 irregular and sinuous contour (Fig. 18, B). These bands undergo 

 still further segmentation (Fig. 18, C), until finally the plasmic con- 

 tents are broken up into numerous small pieces, each containing a 

 nucleus, which ultimately separate and develop into gametes (Fig. 18, 

 D). The method of division in these portions referred to in Fig. 18, 

 B, C (Klebs continues), appears to consist in a constriction, progress- 

 ing from one side, but not entirely completed, since the individual 

 parts remain in communication ; yet direct observation shows also that, 

 in the plane of division, a colorless line or furrow is frequently present, 

 which gives the impression that the constriction may proceed from 

 within. The same principle operating in the segmentation of the 

 bands or pieces obtains also in the earlier cleavage of the whole 

 plasmic layer of the cell. There is from beginning to end a progres- 

 sive condensation, but the process that plays the chief r61e is concealed 

 from observation. 



