116 BIOLOGY: GENERAL AND MEDICAL 



more or less distant from one another, and connected 

 only by delicate protoplasmic filaments, sometimes 

 forming compact masses. Any individual seems able 

 to withdraw from the union to found a new colony, 

 though this is usually observed to follow division of one 

 of the members whose descendant, assuming a modified 

 form, escapes from the cell capsule in which it was born 

 and deserts the colony. 



In many cases the primary purpose of cell association 

 appears to be founded upon reproductive advantages 



FIG. 39. Magosphsera planula, a ciliated colonial protozoan. (After Haeckel.) 



thus afforded, for one of the first structural specializa- 

 tions to be observed among the lowly forms of life con- 

 sists of special reproductive cells. 



We find that with the exception of the most lowly 

 organisms of both animal and vegetable forms, repro- 

 duction takes place either exclusively or most advan- 

 tageously when the newly formed individual or indivi- 

 duals receive substance directly or indirectly from two 

 individuals of its kind (amphimixis). Emphasis will 

 be laid upon the importance and significance of this in 

 the chapter upon Reproduction. 



