IOC EIGHTEENTH REPORT. 



Both body and germ cells have each their own individual gelatin- 

 ous envelopes, which may be made out by careful manipulation of 

 the high ]iower of the microscope. Each kind of cell has two 

 flagella projecting through the common gelatinous envelope. Each 

 cell has a red eye-spot and numerous green chloroplasts. 



Reproduction takes place asexually by the repeated division of 

 the germ cells or parthenogonidia to form daughter colonies. A 

 complete series of developmental stages comprises 2, 4, 8, 16, 32, 

 and G4 cell stages. The daughter colonies eventually break out 

 from the enclosing parental envelope. At the time of its escape 

 each daughter colony consists of either G4 or 128 cells all of the 

 same size; the germ cells are later differentiated by an increase in 

 size. At the time of the escape of the daughter colonies, the body 

 cells of the parent degenerate and die. 



Though Pleodorina is undoubtedly a protozoan colony, it is in 

 some respects transitional to the metazoa and for purposes of com- 

 jjarison with the metazoa it may be regarded as an individual 

 organism. Pleodorina is the simplest and most primitive organism 

 showing a separation or segregation of body cells from reproductive 

 cells; in other words, it is the simplest organism showing differ- 

 entiation of structure and division of labor between somatic and 

 germ cells. It is also the simplest organism which clearly under- 

 goes natural death; but it is only the body cells which die, while 

 the germ cells live to give rise to a new generation of bodies and 

 germ cells. In Pleodorina the fundamental biological principles 

 of segregation of the body plasm and continuity of the germ plasm 

 are exemplified in their simplest form, without the complication 

 of sexual reproduction such as is found in Volvox. 



The value of such a type for elementary classes in biology is 

 obvious. Pleodorina readily falls in place in the series leading 

 from the simplest colonial flagellates, such as Gonium and Pan- 

 doriua, to Volvox. Unfortunately Pleodorina is of rather excep- 

 tional occurrence, but since when found at all it is likelj^ to be 

 present in abundance, a supply sufficient for several years may be 

 preserved. Material fixed in weak Flemming's solution (one part 

 of strong Flemming to three parts water) for twenty-four hours, 

 then thoroughh^ washed and preserved in 5% formalin, retains the 

 natural appearance and form of the colon}', and for class use is 

 almost as good as fresh material. The writer has prepared Volvox 

 in this way and found it in good condition for about six years, 

 after which the finer details of structure were lost. 



Zoological Laboratory, State Normal College, Ypsilanti, Mich. 



