FORMATION OF ANIMAL AGGREGATIONS 45 



als of the lot. In both cases the aggregation forms after a large num- 

 ber of apparently random movements in which the individuals react 

 to the others present in much the same way that they do to pieces of 

 glass rods or to eelgrass. Once formed, these aggregations tend to 

 move together and so to form a more compact bunch. This may 

 smack of a social tendency, although similar behavior is shown to 

 occur when isolated individuals are adjusting themselves to the in- 

 equalities found in a tuft of eelgrass or a loose pile of glass rods. 

 These bunches of Opiiioderma are formed in the same general manner 

 already described for land isopods. 



Such behavior as that of the land isopods or of these starfish is 

 obviously to a large extent conditioned by the reactions of the ani- 

 mals to their physical surroundings. In the absence of elements usu- 

 ally found in the normal physical environment, animals may so react 

 to each other as partially to substitute for the normal environment; 

 that is, other individuals may take the places usually occupied by 

 non-Hving environmental items. Two types of explanation have been 

 advanced for this kind of phenomenon, one of which implies some 

 innate social tendency. The other explains such aggregations in more 

 objective terms. 



THE FORMATION OF CELL AGGREGATES 



Roux (1894), a distinguished experimental embryo logist, observed 

 that when cells of the frog's egg are shaken apart during early stages 

 of cleavage and placed in water only a short distance apart, they 

 slowly approach each other until they come in contact. He termed 

 such cell behavior "cytotropism." In normal development this tend- 

 ency acts to help keep the cells close together in a compact mass. 

 Later Wilson (1910), Galtsoff (1925), and Child (1928), among 

 others, have observed the behavior of dissociated tissue cells of 

 sponges and hydroids. Some of these thoroughly dissociated cells 

 move about and collect in cell aggregations which under proper con- 

 ditions regulate into new organisms. Galtsoff for sponges and Child 

 for the hydroid Corynwrpha have concluded that these cells come 

 together as a result of chance movements on the part of certain cells 

 which incidentally collect other cells as they move and by chance 



