576 VISIBLE CONSTRUCTIVE ACTIVITY IN PROTOPLASM. 



form of the body is retained for a time, and the ellipsoid increases equally on all 

 sides. When it has once attained a certain size, the protoplasm bulges out at a 

 particular place, and a wart-like protuberance arises at the periphery, at first 

 exceedingly small, but gradually increasing in extent, and at length reaching the 

 size of the ellipsoid from which it was produced (ef. vol. II. figs. 371 ^ and 371 *). 

 To say that the cell- wall of the yeast-cell protrudes or grows out, and that proto- 

 plasm immediately enters into the protuberance, is not a correct account of this 

 process. The cell-wall here is only passive : it projects beyond the periphery of the 

 ellipsoidal parent-cell only because it is itself the skin of the protoplasm pushing 

 its way out at that point. From one yeast-cell two outgrowths may arise at 

 diff'erent places, and each of them, when it has once reached a certain size, may 

 again form protuberances. In this way the yeast shapes itself into a structure 

 which strongly recalls to our mind the Prickly Pear in outline (c/. vol. II. fig. 

 371 ^). When the protuberance has grown to an ellipsoid, equal in size to that from 

 which it originated, the slightest pressure is sufficient to disconnect the two, and 

 to separate the individual members of the irregular opuntia-like chain. Even 

 without any external stimulus the cells separate, as may be well observed in 

 brewers' yeast (Saccharomyces cerevisice), which of all the species of yeast has 

 been most investigated. 



The formation of yeast by the development of a cell-wall as a partition between 

 two adjoining cells reminds one of the division of cells which has now to be 

 described as the third formative process connected with growth. The division of 

 the cells is always accomplished in the following manner: — The protoplasm, 

 inclosed in its cell-wall, develops a partition in its interior by which it becomes 

 divided into two halves, and the cell-space into two compartments or chambers. 

 In some plants the sister-cells produced in division separate from one another, the 

 partition-wall becoming completely split, but in most cases the neighbouring cells 

 remain connected, and then in each of these the same process is repeated; in this 

 way arise multicellular structures, that is, aggregates of cells. 



A separation of the two cells arising from a division, due to the splitting of 

 the intervening wall, is observed in the Desmidiese, those small green aquatic 

 plants, of which two species are represented in figure 25a, i, k. Although the 

 Desmidiese consist only of a single cell, their multiplicity of form is considerable. 

 We have cylindrical, semilunar, tetrahedral, stellate, and disc-shaped forms in inex- 

 haustible variety, often occurring in a restricted area, and forming a gay assemblage 

 like the various herbs growing in a meadow. The cell of each species, however, 

 adheres with wonderful tenacity to its plan of construction, and always develops 

 only to a definite size. When once this size is attained, and after the cell has re- 

 mained unaltered in form for a time, a noticeable change begins to take place. The 

 central portion of the cell (which is constricted in all species) quickly elongates and 

 expands. The protoplasm then develops a dividing-wall, and two cells are now 

 produced from the one. These remain connected for only a little while; the inter- 

 calated cellulose wall splits; the two cells separate, and each forthwith assumes 



