302 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



to the development of colonies out of isolated cells. Such colonies are observed, 

 for instance, in the ciliate Zoothamnium alternans ; but here, in contrast to 

 the most primitive tissues, a differentiation in function has taken place between 

 different members of the colonies, as Summers has shown. The apical cell 

 exerts an inhibiting effect on neighboring cells. When the apical cell is cut 

 away, a formerly subordinate cell becomes dominant and assumes the genera- 

 tive function of the apical cell. But the latter may also exert a stimulating 

 effect on the other members of the colony. If it becomes an exconjugant, it 

 induces the first three or four branches below its own level to divide pre- 

 cociously and so actively that each branch develops almost as an individual 

 colony. In such a colony evidently a complex tissue equilibrium exists, but 

 whether this equilibrium requires a strictly autogenous relationship between 

 the various members of the colony is not certain. 



3. The next higher type of tissue formation is found in sponges. Some 

 complications are added here to the primary factors observed in amoebocytes. 

 H. V. Wilson, who first separated sponge cells experimentally, was able to 

 observe that these cells later united again with one another, forming aggregates 

 from which, under favorable conditions, complete sponge organisms devel- 

 oped. More recently Galtsoff noted that it is the archaeocytes which play the 

 principal role in the agglutination of sponge cells, and that they resemble very 

 much in their behavior the amoebocytes of Limulus. As in amoebocytes, so 

 also in sponges the migrating cells happen to meet other cells of the same kind 

 in the course of their movements, and whenever such a chance meeting takes 

 place the cells stick together. In both cases there is the same lack of an orient- 

 ing force which leads to the tissue-like agglutination of cells. However, sub- 

 sequently some differences develop between the behavior of amoebocyte tissue 

 and sponge-cell aggregations. In the latter, a secondary detachment and 

 migration of cells in a centrifugal direction does not occur as it does so often 

 in the former; instead, they now spread out on the surface on which they 

 are resting, in a way comparable to the extension which is such a common 

 occurrence and which we have analyzed in amoebocyte tissue. In both these 

 types of cells the process of extension can be considered as a pathological 

 modification of amoeboid movement. However, subsequently, the aggregates 

 of sponge cells, provided they are sufficiently large and contain cells of a 

 certain type, may change into normal proliferating sponges, while from 

 amoebocyte tissue more complex formations may develop merely as a result 

 of secondary, often degenerative changes which lead to the production of 

 paraplastic structures. The mechanisms, in both instances, underlying the 

 primary agglutination and the development of stickiness in the hyaline ecto- 

 plasm, which latter precedes the agglutination process, are related to the factors 

 concerned in the production of pseudopods and in the extension of the cells. 

 Temperature, osmotic pressure and hydrogen ion concentration seem to affect 

 amoebocytes and archaeocytes in a similar manner; for instance, alkali in- 

 creases the tendency of both kinds of cells to agglutinate. But as far as the 

 effect of salts, and of ions composing them, on tissue formation is concerned, 

 the reactions of archaeocytes apparently correspond more closely to those 



