EXERCISE V 



CELL AGGREGATION, DIFFERENTIATION, AND 

 DIVISION OF LABOR 



A. AGGREGATIONS OF CELLS 



When a unicellular animal divides, two daughter animals are formed 

 which usually separate from one another. Thus one-celled organisms 

 are always of small size, in most cases invisible to the unaided eye. Ani- 

 mals that reach visible dimensions almost always consist of more than one 

 cell. Increase in size, in these, is due to accumulation of the cells as they 

 divide. A group of cells derived from one cell by division may be called 

 a cell aggregation. Various types of aggregation are described below. 

 Try to discover their fundamental differences while this study is in prog- 

 ress, and arrange them in a definite scheme at the end of the exercise. 



1. Epistylis is a colonial protozoon usually found attached to small 

 freshwater animals. Examine demonstrations of stained specimens. 

 Note the method of branching. The oval-shaped bodies at the ends of 

 the branches are the individuals of the colony, and each one consists of a 

 single cell. Note the nucleus; what is its shape? Are the cells alike, or 

 distinctly different? 



Reproduction in Epistylis takes place by a simple division of an indi- 

 vidual into two daughter individuals which remain attached to the colony 

 by independent stalks. 



Sketch a small colony. 



2. Carchesium and Zoothamnium are other colonial Protozoa. 

 Observe living specimens if obtainable, otherwise omit this section. 

 Study a colony in a salt cellar with a dissecting microscope. Note 

 that each individual is attached to the end of a long contractile filament 

 or stalk. Can it retract itself independently of its fellows? The con- 

 tractile element is absent in Epistylis. Note the result of touching one or 

 several individuals with the point of a needle. 



In Carchesium and Zoothamnium as in Epistylis the cells are inde- 

 pendent of each other and each cell elaborates its own stalk, and carries 

 on the metabolic processes, movements and reproductive functions 

 independently of the colony as a whole. 



Write out your observations on the living Carchesium. 



3. Pleodorina californica is a free-swimming organism found in fresh- 

 water ponds. Study preparations of stained specimens. Note the small 



23 



