574 



GENERAL PHYSIOLOGY 



to form a row or a surface. This is the case here. The largest 

 cell-communities composed of like cells that are known among the 

 Protista are those that are related to the Algce among the plants. 

 They are either fibrous, such as the Confervce (Fig. 276), or folia- 

 ceous, such as the large Ulvacece. In the latter, cell is attached to cell 

 to form a flat surface, so that the part of the cell-surface that is 

 free, and the part that borders upon its neighbours are the same 

 in every cell ; thus all cells are under like external conditions. But, 

 if the cells proceeding from the division of one cell do not all 

 remain under like external conditions, and if the cells do not perish, 

 in time differences must appear. This condition is realised in the 

 formation of every cell-community the component cells of which 

 are not arranged in a flat surface, but are distributed in all direc- 

 tions as solid complexes. Here the cells 

 that lie in the interior of the community are 

 under wholly different external vital condi- 

 tions from those at the surface. As a result 

 of this they must form a contrast to the 

 latter, both morphologically and physiologi- 

 cally ; in other words, differentiation and 

 division of labour result. The simplest 

 examples of this are likewise met with in 

 certain forms of the Protista, which form 

 such an extremely interesting transition- 

 stage to the cell-communities of the plants 

 and the animals. Such an organism is the 

 Protospongia Hceckelii (Fig. 277), a colony of 

 flagellate Infusoria, which as regards histo- 

 logical structure has a certain similarity to 

 the lowest sponges. Upon the surface of a 

 gelatiiious mass sit numerous, cup-shaped, 

 flagellated cells, while in the interior of the 

 mass there are many amoeboid cells without 

 flagella. Here, therefore, is a differentiation of the cells living in 

 the interior as compared with those living upon the surface, which 

 is extremely marked and the cause of which is at once evident. It 

 is especially interesting in connection with this organism that the 

 differentiation exists only so long as the causes exist. The 

 amoeboid cells of the interior, for example, have the power of 

 wandering to the surface, and in this case they likewise develop 

 into cup-shaped flagellated cells. In these lowest forms of the 

 differentiated cell-community, therefore, the individual cells still 

 possess in the highest degree the capacity of changing into other 

 forms. 



Differentiation of the cells by adaptation to the external con- 

 ditions afforded by different positions, which is only barely indicated 

 in the Protista, is the fundamental principle in the construction of 



Fig. 276."iSpi?'Oj;j/ra, a multi- 

 cellular, fresh -water Alga. 

 A, Piece of a multicellular 

 thread. B, Single cell. In 

 every cell the chlorophyll- 

 body winds spirally along 

 the inside of the wall. 



