NOTICE OF THE GENUS TETRAPEDIA, ETC. 359 



cell presents a lanceolate outline, the extremities acute (fig. 



10). 



The quadripartite condition is regarded by E-einsch as a 

 "state of division ;" but he does not figure or refer to the simple 

 or undivided state : it is not clear, indeed, how the carrying 

 onwards of the incisions to completion, even until it should 

 involve entire separation of the segments, would result in the 

 formation of a secondary set of quadrate cells, like the young 

 form — if pushed through and through the quadrate cell would 

 simply become cut into four triangular ones — two sides 

 straight and one with the two sinuses. 



One might, suggestively, however, put the following pos- 

 sibility. Assuming the incisions carried on till four distinct 

 younger cells were produced, of course of the triangular 

 figure, they each would be equal to one fourth the super- 

 ficial dimensions of the original cell, and they would resemble 

 a moiety of a similar cell of one half its superficial dimen- 

 sions, and as if separated from its assumed fellow diagonally. 

 Now, we might suppose each of them capable of growing a 

 new half of triangular form, the obtuse-angled prominence 

 taking the place, as it were, of a kind of puncium vegetationis, 

 that is (so to say) as if the cell were stretched or pulled by 

 some force to double its size, the little prominence always 

 remaining the aj^ex, until a quadrate figure were attained ; 

 it would, of course, likewise be requisite to suppose a general 

 increase in size of the whole cell thereupon, or pari passu ; 

 they occur, however, of varying sizes. It would further be 

 necessary to suppose a new incision to originate at each 

 angle, and the central protuberance to be developed at each 

 side, producing the two sinuses. Were all this accomplished 

 in each of the separated cuneate segments, four new similar 

 cells to that figured would be the result. This, however, is 

 but quite conjectural, and it seems to remain quite uncer- 

 tain as to how the cells grow and divide into new cells. 

 The few examples I have seen were similar to those figured, 

 that is, deeply incised with the four broadly cuneate seg- 

 ments, the specimens slightly diflfering in dimensions, and 

 showed no tendency to any further alteration. There is a 

 certain amount of possibility of this form being taken for a 

 minute Pediastrum (Meyen) ; but apart from the colour of 

 the contents there is no Pediastrum with similar outline of 

 the cells, and, indeed, as regards the examples hitherto seen, 

 the common union of the segments at their bases prevents 

 any confounding with a Pediastrum, which is composed of 

 distinct but closely juxtaposed cells, held together as a 

 frond or *^ coenobium" by a kind of hyaline " intercellular 



