152 
W. Bernard Crow. 
Contractile vacuoles often more 
than two, sometimes very 
numerous, distributed over the 
periphery of the cell. 
Chloroplast an indefinite parietal 
sheet or net, often with numer¬ 
ous pyrenoids. 
Contractile vacuoles usually two, 
in anterior position. 
Choloroplast a basin - shaped 
structure, often with few 
pyrenoids. 
Turning to the motile colonial genera of the Chlamydomona- 
dales it is apparent that the coenobia of Goniwn, Pandorina , 
Endorina and Volvox are all of the same type. The three latter 
genera pass through a plakea stage in their development during 
which they resemble colonies of Goniwn. However, it has been 
shown by Harper [10] that the plate-like form of such colonies can 
be explained on purely physical grounds. No doubt the evagina- 
tion of the plakea is also due to mechanical factors. In any case 
the phenomena of development cannot be used safely for classifica- 
tory purposes since Powers [16] has shown that in Volvox 
Weismanniana many of the colonies are turned inside out, so that 
the originally inner surface comes to lie outside, whilst in others 
of the same species this is not the case. Finally the resemblance 
in general type of these coenobia is not indicative of affinity, for 
similar colonies are by no means rare in other algae and Protozoa, 
whilst the blastula of the Metazoa is planned on the same lines. 
One of the characteristic features of Volvox is that the power 
of reproduction is limited to a few cells in the posterior region of 
the ccenobium, whereas, in the three genera mentioned above, all 
the cells participate in this process. The connecting link is 
supposed to be found in species of the so-called Pleodorina [see 
5, 11] where half the cells are vegetative (P. californica), or there 
is merely a group of four sterile cells (P. illinoisensis). The latter 
form has been shown to be a variety of Eudorina elegans [8, etc.] 
there being all transitions between the typical forms of the latter 
and the original P. illinoisensis. 
In material from, an unknown locality 1 I have observed varia¬ 
tions in Pandorina morurn bearing the same relation to the typical 
forms of that species as Pleodorina illinoisensis bears to Eudorina 
elegans, except that the shape and size of the colony are not 
affected. In one sample some forty colonies in the reproductive 
condition were examined. Only fifty were absolutely normal 
1 Obtained from Messrs. Flatters and Garnett, Manchester. 
