KOFOID: DEVELOPMENT OF LIMAX. 93 
into a fresh-water environment, still retains the free-swimming larval 
stage characteristic of marine forms. It has acquired, however, the 
"primitive segmentation cavity” found in the fresh-water Lamelli- 
branchs, but not definitely known to be present in the marine forms, 
Lankester (74) does not refer to the cleavage cavity of Pisidium, nor 
does he figure it except in comparatively late stages of development. 
Von Jhering (76) speaks of tho three or four small cells in Cyclas, 
whose progeny grow around the solid mass of the two large cells, and 
of the later appearance of a cavity in the centre of this mass. Ziegler 
(85) finds a cavity in the thirteen-cell stage of Cyclas, but indicates no 
cavity in the two earlier stages that he figures. 
The latest, and by far the most important, contribution to our knowl- 
edge of the cleavage cavity is that of Stauffacher (93) upon Cyclas 
cornea. The formation of a “true” cleavage cavity takes place at the 
thirtecn-cell stage by the gradual elevation of the cap of ectoderm cells 
from the maeromere to which they had been closely applied, resulting 
in the development of a sharply defined space between the macromere 
and its derivatives. This cavity persists and increases in size until it 
ultimately becomes the relatively very large cavity of the blastula stage. 
In addition to this cavity, which he regards as persistent from the 
thirteen-cell stage on, Stauffacher finds in the two-cell stage a structure 
which he regards as similar to that observed by Flemming in Anodonta 
and by Rabl in Unio. He describes it as a disproportionately large 
space, entirely unstainable, ¿n the smaller of the two spheres, exactly in 
the region where they are in contact. The cavity is filled with a fluid 
free from granules. The protoplasmic part of the cell, which forms the 
peripheral layer and contains the nucleus, merges very gradually into 
this fluid-filled space. On the side of the macromere this space is 
sharply and definitely limited. It seems from his description that this 
space is regarded by him as lying in the smaller cell, i. e. intracellular, 
though he does not distinctly designate it as such. The interpretation of 
this space and its later history are best given in his own words (loc. cit., 
p. 211): “Es fällt bei Cyclas nicht schwer, den unumstosslichen Beweis 
zu erbringen dass der genannte helle Raum in der That nichts mit einer 
Furchungshöhle zu thun hat. Dieselbe körnerlose Partie nämlich, die 
wir auf dem zweizelligen Stadium antreffen, ist zwar auch auf dem drei- 
zelligen Stadium noch vorhanden, aber schon bei der Bildung der vierten 
Furchungskugel wird sie bedeutend reduziert und verschwindet schliess- 
lich ganz. Dagegen entstehen im weiteren Verlauf der Entwickelung 
zwischen der grossen Mutterzelle und ihren jeweiligen letzten Abstam- 
