On Pleodorina Illinolsensis. 275 



five circles, two of which are polar and contain four cells 

 each, while eight cells are found in each of the remaining 

 three circles, one of which is equatorial and the other two lie 

 between the latter and the polar circles. The cells resemble 

 those of Eudorina in that they are situated in the periphery 

 of the hyaline gelatinous matrix and are not closely crowded 

 together, the degree of separation depending upon the age of 

 the colony and varying considerably in different cases. 

 Their inner ends do not approach the center of the colony as 

 is the case in Pandorina. No trace of any protoplasmic 

 connection between the cells of a colony could be detected in 

 the living organisms, nor in material killed in formalin or in 

 chromo-aeetic acid and afterwards stained in fuchsin, hema- 

 toxylin, or Bismark brown. Specimens treated by Zograf's 

 method (1% osmic acid followed by 4% crude pyroligneous 

 acid) or by 1% osmic acid followed by picrocarmine, showed 

 no connection between the cells. 



The colony is surrounded by a common gelatinous sheath 

 (sh.) increasing in thickness (3.5 to 12 /<) with the age of the 

 organism. This membrane or sheath is of equal thickness in 

 all regions and consists of two parts : an outer, thin, denser, 

 more highly refractive layer (o.l.) ; and an inner homogeneous 

 one (/./.), which shows no traces of the concentric structure 

 found in Pandorina. It is within this latter layer that 

 the increase in thickness takes place in the older colo- 

 nies. It is limited centrally by a thinner and less highly 

 refractive layer (m.m.) which encloses the common matrix 

 (m.) in which the cells of the colony lie. Frequently among 

 the older organisms there occur upon the posterior end of the 

 colonies blunt, pseudopodia-like protuberances (PL XXXVI., 

 Fig. 4) of the sheath, of irregular form and of no constant 

 number. Their position and the fact that they are often, 

 though not always, found in old colonies from which some of 

 the daughter colonies have already apparently escaped, sug- 

 gest that they may mark the place of exit of the young indi- 

 viduals from the parent. Similar protuberances were ob- 

 served upon Eudorina and Pandorina, under similar con- 

 ditions, in the collections in which the Pleodorina under 



