Carter \ — Studies on the Chloroplasts of Desmids. IV . 313 
the chloroplast (Figs. 68-70), but the constriction of the chloroplast extended 
over two days. The complete division of the chloroplast was observed later 
in Micrasterias denticulata , in which the length of time between the stage 
represented in Fig. 69 and the final breaking of the drawn-out connecting 
thread was about sixteen hours. In their natural surroundings, however, 
it is most likely that the division of the chloroplast takes place far more 
rapidly than in specimens kept under observation under more or less 
unhealthy conditions. For in fixed and stained material one often encounters 
young individuals whose new semi-cells are not yet fully formed, yet whose 
chloroplasts have nevertheless completely divided at the isthmus. 
It will be noticed that the actual division of the chloroplast differs 
somewhat from that previously described for Closterium and Hyalotheca. In 
both these genera there is a pinching-in of the chromatophore, and under 
the influence of the nucleus this furrow becomes deeper and deeper until the 
two halves are completely severed. In all the forms examined in this work 
such a pinching-in was not observed, the two halves of the chloroplast 
apparently pulling themselves apart, the connecting strand between them 
becoming thinner and thinner until it finally broke. 
Owing to the dense nature of the cell-contents in most cases, and the 
large quantity of starch contained in the chromatophore, the behaviour of 
the pyrenoids could not usually be traced in the living condition, but in 
Cosmariam subtumidum it was possible under high magnification to dis- 
tinguish the pyrenoids, and also to keep them under observation during the 
division processes. 
In this species the single central pyrenoid, soon after the beginning of 
the budding of the chloroplast, begins to elongate slightly at its lower end 
(Fig. 71). The starch-grains rearrange themselves rapidly, and those at 
the lower end of the pyrenoid seem to form a small loop which 
doubtless contains a small globule budded off from the pyreno-crystal 
(Fig. 72). The latter cannot be seen, however, because of the great 
refractivity of the starch-grains. The small pyrenoid thus budded off from 
the original one apparently increases in size (Fig. 73), and the starch-grains 
arrange themselves so as to cut it off from the old one (Fig. 74). The 
process takes place very rapidly, ten to fifteen minutes sufficing for the com- 
plete division. During the formation of the new pyrenoid, the latter is 
gradually transported with the budding chloroplast into the young semi-cell 
(Figs. 71-4), but the two pyrenoids remain connected by means of starch- 
grains for a considerable time (Fig. 74). 
In most of the species examined with more or less flattened cells, the 
chloroplast entered the young semi-cell through the isthmus as a somewhat 
bilobed protuberance (Fig. 62), although this form was sometimes quickly 
lost as the chloroplast streamed more and more into the new semi-cell. In 
the case of cells triangular or pentangular in the end view, and having nor- 
