REPRODUCTION IN THE UNIONID^ 
93 
made. Both contain embryos in an early cleavage stage, onl}^ a 
few of which, however, are represented. The actual sections show 
the spaces closely packed with embryos. In fig. 31, which may 
be taken as typical of the condition existing in the Heterogense, the 
tubes are not only enormously enlarged, but their walls are rela- 
tively thinner, and the whole marsupium has a more membranous 
appearance than in the Quadrula and Unio types. 
The glandular epithelium is found chiefly on the surfaces of the 
interlamellar junctions, and in some species is much more highly 
developed and more extensive than in others, often causing the 
surface to be thrown into irregular folds and ridges. This is very 
pronounced in Unio and Quadrula. When highly magnified, as in 
fig. 32, the epithelium, resting upon a base of connective tissue 
and smooth muscle fibers, is seen to be composed of greatly swol- 
len cells, whose vacuoles are filled with a clear mucus-like colorless 
fluid. Scattered among the large cells and seemingly often lying 
within the vacuoles, are seen several smaller and darker nuclei 
which are undoubtedly the nuclei of leucocytes. In fact, there 
can be little doubt that the epithelium becomes infiltrated with 
wandering blood cells from the underlying sinuses in the inter- 
lamellar junctions. Many indications are present that seem to 
show that these cells actually wander through the epithelium into 
the cavities of the water-tubes but what their ultimate fate is, if 
this be the case, we are as yet unable to say. Possibly they may 
be ingested by the glochidia and used as food. 
STRATIFICATION OF UNFERTILIZED EGGS 
It not infrequently happens that eggs pass into the marsupium 
without being fertilized, and remain there throughout the period 
of embryonic development, as one may find them in the same gill 
with fully formed glochidia. In some individuals we have found 
every egg in the marsupium in this condition. Such eggs have 
been encountered, howev^er, only in summer breeding species and 
they seem to be especially common in the genus Quadrula, nearly 
every gravid female of which has been found to contain at least 
some unfertilized eggs. After remaining in the marsupium for 
