NEW RHABDOCCELE COMMENSAL WITH MODIOLUS 379 
bryos instead of two in the same capsule (fig. 28) . So far as such 
cases have been studied it appears that two of the three are alike 
and smaller than the third, which should be the case unless the 
original division of the first cell or the mass of cells were three- 
fold, an occurrence certainly not to be expected. Often a young 
worm is seen not accompanied by its twin. These cases, as a 
rule, are the result of the rupture of the embryonic capsule and 
the escape o^ the young into the maternal mesenchyme where they 
can wander about freely. It happens, however, that, while it is 
the rule that two embryos develop in each capsule, only one may 
at times develop. This is the interpretation which I think must 
be given a case where a single young worm is still enveloped in the 
capsular covering (fig. 1, n). 
The cleavage of the mass of cells into two masses is preceded 
by the arrangement of the large and the small cells in different 
ways. In some cases the small cells are at the opposite poles and 
the large cells in the middle in which case the division takes place 
through the mass of large cells (fig. 1, r). Or the large cells may 
be at the opposite poles and the small cells in the middle, the 
division then taking place through the latter (fig. 3, e and/). In 
other cases it would appear that at the first division of the germ - 
inal spot, the resulting cells have moved apart, each building up 
a morula-like mass of cells. These two masses of cells remain 
for a long time separated from each other by an intervening mass 
of yolk (fig. 22). Such cases as young worms with one or more 
masses of yolk in the same capsule are thus explained. Some 
cases were observed where there was nothing but a mass of large 
cells within a capsule, the yolk having been entirely absorbed 
(fig. 25). What is the ultimate fate of such cell masses I do not 
know. 
While I did not contemplate the study of such minute structures 
as nuclear elements, some of the material was prepared with 
considerable care. Chromosomes are clearly defined, and a few 
very large centrosomes were noted. What was interpreted to be 
a polar body was noted in one case. This was a cell w^hich lay 
inside the egg capsule at the junction between the approximated 
anterior ends of the embryos. 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 9, NO. 2. 
