1912.] The Life-history of Cladothrix dichotoma (Cohn). 



349 



cytoplasm may subsequently be seen in the surrounding fluid. These 

 lumps must serve as a valuable source of food to the other micro-organisms 

 in the water. 



Hoeflich(12) in p. 61 speaks of vacuoles as though they were obvious 

 constituents of the cell ; as he does not mention the presence of oil and 

 worked with artificial cultures, I think it probable that his vacuoles were 

 either oil drops or empty cavities in the cytoplasm. On several occasions 

 I have observed apparent vacuoles in the stained cells of artificial cultures, 

 but as these might just as well be referred to the spaces originally 

 occupied by the food material I have refrained from drawing any conclusions 

 from the fact. 



False Dichotomy. — The characteristic tree-like appearance of Cladothrix 

 (fig. 3) is assumed only when the water is practically stationary. A cell 

 slips out sideways through the mucilaginous sheath and grows into a thread, 

 the mucilage holds the branch and parent together, and so it appears as if 

 true branching had taken place. In fig. 16 is shown a " branch " attached 

 to the parent thread. At the point of attachment there were no indications 

 of the existence of two separate sheaths. As the cells which give rise to 

 the daughter-thread slip out through a very soft sheath, such is not to be 

 expected, and when later the mucilaginous sheath hardens, at the point of 

 contact of the daughter- with the parent-thread, the sheath would harden as 

 one piece. When the conditions are not restful, the falsely dichotomous 

 growth is not possible, for the mucilaginous sheath has not sufficient adhesive 

 power to prevent the escape of the liberated cell, but, if the conditions are 

 restful, adhesion is possible, and, when the sheath later hardens, the escape 

 of the daughter-threads is rendered quite impossible, even if the water 

 becomes subsequently somewhat less restful. It is, therefore, only under 

 exceptional conditions that the Cladothrix of the text-books is formed. 



Cell-division. — The stages of cell-division can be followed by reference to 

 fig. 15. There is first a slight elongation of the cell, then a slight con- 

 striction in or near the middle, after which a thin transverse membrane is 

 thrown across the cell at the constricted part. The constriction gradually 

 deepens and the membrane becomes thicker ; this is followed by a drawing 

 apart of the daughter-cells, after which each elongates until the adult form 

 is reached. The process is thus essentially the same as in the Bacteriaceae 

 Stained preparations sometimes show that the separated cells are connected 

 by a mucilaginous thread (fig. 17). ISTo doubt, for a time, a thin thread of 

 protoplasm, running through the mucilage, also joins the cells. 



When the sheath hardens, the pressure and growth of the dividing cells 

 causes a rupture of the sheath at the apex, after which the sheath remains 



2 B 2 



