THALLOPHYTA. 633 



gametes, though not sexually differentiated inter se, since conjugation is apparently 

 possible between any two, whatever their relative size, may perhaps be considered 

 as male in relation to the largest. 



It is probable that fully-differentiated male and female gametes arose from 

 forms such as we find in Pandorina, by the suppression of the intermediate sizes, 

 the smaller and more active gametes taking on the function of actively seeking out 

 the larger passive individuals, which on their side contribute practically the whole 

 of the stock of food required by the zygote in germination. In correspondence with 

 this we find the chromatophore (chlorophyll-corpuscle), which may be considered as 

 the specially food-producing organ of the algal cell, much reduced and eventually 

 functionless or absent altogether in the more highly differentiated male gametes 

 (spermatozoids). 



Eudorina has a colony of sixteen or thirty-two almost spherical cells consider- 

 ably separated from one another, and inclosed in a general investment like that of 

 Pandorina. In the production of daughter-colonies Eudorina resembles the lower 

 forms of the volvocine series, but in the sexual differentiation of the gametes there 

 is a decided advance upon that obtaining in Pandorina. The perfectly passive 

 female gametes (oospheres) hardly differ from the ordinary cells of a vegetative 

 colony, while the active male gametes (spermatozoids) are formed in bundles of 

 sixty-four by successive divisions of similar cells. Here, then, we find the marked 

 difference in size between the two categories of gametes brought about, as it very 

 often is among the Algae (and, indeed, among many other plants and animals), by a 

 marked difference in the number of divisions occurring in their 'respective mother- 

 cells. The present case in which strictly comparable cells on the one hand directly 

 give rise to eggs, and on the other divide to form sixty-four spermatozoids each, is 

 rather extreme, but we have already met with a similar case in a species of 

 Ghlamydomonas. 



Each spermatozoid of Eudorina is club-shaped, with a colourless pointed 

 anterior end bearing two flagella and possessing an eye-spot, and a yellowish thick 

 posterior extremity representing the (reduced) chlorophyllous portion of the tj^pical 

 volvocine cell. The spermatozoid bundle (male colony) escapes from its mother-cell- 

 membrane, and swarms as a whole towards a female colony. On reaching the latter 

 the spermatozoids get their flagella, become entangled in the thick mucilage, and 

 rapidly separating from one another, M^orm their way into the female colony. Some 

 succeed in fusing with the individual female gametes, and each zygote thus formed 

 will eventually give rise to a new Eudorina colony. 



A form recently discovered almost at the same time in three different States of 

 North America, and known as Pleodorina, shows an important difference from the 

 types we have hitherto been considering. Each spherical colony consists of about 

 128 cells, but not all of these are capable of producing daughter-colonies. This 

 power is confined to those cells which occupy the posterior half or two-thirds of the 

 sphere (it should be explained that the colony moves forward in relation to a 

 definite axis). The smaller anteriorly-placed colls are thus purely vegetative in 



