1018 ELLIOT ROWLAND DOWNING 
SPERMATOGONIAL MACROMERES 
The giant spermatogonium contains a very large nucleus and 
a prominent nucleolus (fig. 17). When shed into the body fluid 
the cell undergoes an interesting development. It divides un- 
equally, producing one large and one small cell, (fig. 18). The 
small cell next divides, giving a three-cell stage. Unequal divi- 
sion of the large cell then occurs, producing a four-cell stage seen 
in polar view in fig. 19. It is so evident that these early stages 
in the division of the giant spermatogonia are at least roughly 
similar to the cleavage stages of the egg in Arenicola that we may 
modify the nomenclature of the latter to describe clearly the for- 
mer. Just the order of cleavage of the four cells I have been 
unable to determine except that the three small cells divide be- 
fore the large one, differing in this respect from the division of the 
egg blastomeres. When all have divided we have an eight-cell 
stage consisting of four large cells, the spermatogonial macro- 
meres, and four small cells, the spermatogonial micromeres. 
The position of the cells (figs. 20, 21), shows that the third divi- 
sion has evidently been a dexiotropic one as it is in the egg cleav- 
age. Up to the sixteen-cell stage it is reasonably certain (and 
I think for at least one additional cleavage) that the cell lineage 
of these giant spermatogonia is homologous to that of the egg. 
Spermatogonial blastulae and gastrulae form much as in egg 
development. The figure of the sixteen-cell stage, (fig. 22) 
as indeed all these figures of the cleavage of the giant spermato- 
gonia, are camera lucida drawings done with exceptional care, 
under a one-sixth inch objective and a one-half inch ocular at the 
level of the table. It is needless to multiply sketches as they 
would simply be duplicates of the admirable figures already given 
by Child for the cleavage stages of the egg. 
Shortly after the sixteen-cell stage the macromeres disappear 
from the surface and migrate into the segmentation cavity. If 
one slightly crush the spermatophores in the body fluid under a 
cover glass, the great majority will show the blastophore exuding 
from the center of the spherical masses, yet a few will show four 
to six rather large cells which escape from the cavity. This is 
