Dr. C. A. Kofoid on Pleodorina illinoisensis. 141 



biflagellate cells. The shape is quite constant, occurring in 

 the youngest colony and continuing throughout the asexual 

 cycle until the daughter colonies abandon the gelatinous 

 matrix of the maternal organism. Among the large number 

 examined only a few specimens were seen which approached 

 a spherical form. Measurements of twelve seemingly full- 

 grown colonies from material freshly killed in 2 per cent, 

 formalin showed a range of 101 to 137 /j, in long diameter and 

 an average of 113 fi. The transverse diameter ranged from 

 84 to 102 fi, and averaged 94 fi. Individuals in which the 

 gonidia have begun to divide show a considerable swelling of 

 the hyaline gelatinous envelope. One specimen containing 

 2- and 4-cell stages measured 178 X 155 (i, and when the 

 young colonies are ready to escape the parent may measure as 

 much as 200x175 /jl. At the time of escape the young 

 colonies measure 46 X 38 fi. The measurements of the 

 colonies approach very closely those given by Butschli (1880- 

 1889, p. 840) for Eudorina, viz. 100-150 fi ; and the colonies 

 of this genus found in association with the form here describsd 

 exhibit dimensions almost, if not quite, identical with those 

 above recorded for the Pleodorina. 



The colony (PI. V. tig. 1) contains, as a rule, 32 cells, 

 arranged, as Henfrey (1856) first noted for Eudorina, in 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 be- 

 tween 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 centre of the colony, as is the 

 case in Pandorina. JSIo trace of any protoplasmic connexion 

 between the cells of a colony could be detected in the livino- 

 organisms, nor in material killed in formalin or in chromo- 

 acetic acid and afterwards stained in fuchsin, haamatoxylin, or 

 Bismark brown. Specimens treated by Zograf's method 

 (1 per cent, osmic acid followed by 4 per cent, crude pyro- 

 ligneous acid), or by 1 per cent, osmic acid followed by picro- 

 carmine, showed no connexion between the cells. 



The colony is surrounded by a common gelatinous sheath 

 (sh.) increasing in thickness (3*5 to 12 fi) 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 



