QUANTITATIVE CELLULAR ANALYSIS OF GROWTH AND REPRODUCTION IN FRESH-WATER PLANARIANS (TURBELLARIA, TRICLADIDA) .1. A CELLULAR DESCRIPTION OF THE INTACT ORGANISM

A better understanding of the mechanisms underlying growth and degrowth in freshwater planarians, and the control systems associated with them, requires a thorough investigation at the cellular and molecular levels. As a first step towards this goal, we have measured the body volume, plan area, number of cells, percentage of different cell types and mean cell area and volume in growing and degrowing organisms from five species of planarians with sexual, asexual, semelparous and iteroparous reproduction. Growth and degrowth were found to be due mainly to changes in the number of cells (hyperplasia), cell hypertrophy playing only a minor role. Total number of cells varied enormously during growth though their percentages showed only slight cell-type specific increases or decreases, these tendencies are reversed during degrowth. Although the five species studied differ greatly in reproductive modalities and life-cycle characteristics, the percentages of the main cell types during growth/degrowth were basically similar. Finally, cell size and temperature showed an inverse correlation in all species studied.