CELL AGGREGATES 



77 



parent; both parent and offspring then reproduce in the same 

 way until a branching tree-like colony results (Fig. 31). 

 Sphaeroidal colonies, finally, differ from the others in that the 

 cells resulting from division remain embedded in a gelatinous 

 matrix secreted by the parent organism (Fig. 24). Such 

 colonies are usually spherical, the constituent cells being ar- 

 ranged about the periphery. When this condition in develop- 



/ 



FIG. 31. An arboroid colony of flagellated protozoa, Codosiga cymosa, Sav. 

 Kent. (From Calkins after Kent.) 



ment is reached it is not a long step to the simplest type of 

 metazoan, and we find cases where there is even a regional dif- 

 ferentiation in the colony. A Proterospongia, for example, is 

 a gelatinous mass with cells arranged about the periphery; 

 when abundantly fed these cells migrate one by one to the in- 

 terior of the jelly mass, losing their flagella in the process. 

 Within the jelly they divide, and the cells then wander back to 

 the periphery to take their place with the feeding cells of the 

 colony. Here then is a temporary differentiation into vegeta- 



