1016 ELLIOT ROWLAND DOWNING 
walls would account for. This is not true for the later stages, 
however, for with the formation of the spermatids a decrease in 
the size of the individual cells accompanies the transformation 
of the spermatids to the spermatozoa. 
Contemporaneously with the change of the spermatids to sper- 
matozoa, or even beginning when the cells are yet spermatocytes 
of the second order, a change in the shape of the mass of cells 
occurs. In A. cristata the spherical mass invaginates in a man- 
ner that forcibly suggests the invagination of some egg blastulae 
to form the gastrulae (fig. 24). The gastrula-like mass remains 
cup-shaped, the mouth wide open, the lips never approximating 
to suggest a closure of the blastopore. Usually before invagina- 
tion is complete, the cup begins to flatten out, becoming saucer- 
shaped, which is the form of the mature spermatophore in this 
species (fig. 25). 
This phenomenon is apparently merely analogous to the process 
of gastrulation in the egg. Presumably the physical relations 
between the cell mass and the surrounding medium happen to be 
such that invagination ensues with regularity in this one species. 
Body fluid freshly drawn by means of a hypodermic syringe shows 
these gastrula-like forms, as do also preparations made by fixing 
the coelomic fluid in a variety of fluids. In other species no such 
thing happens, but the spherical mass of cells merely flattens out ~ 
to form a biconvex spermatophore. There is however in the 
spermatogenesis of this group a phenomenon which is really 
homologous to the segmentation and gastrulation of the egg and 
which will be considered in the discussion of the giant spermato- 
gonia. 
The ripe spermatophore 
As the spermatids transform into spermatozoa the cells elon- 
gate, their long axes at right angles to the surface of the saucer- 
shaped or biconvex mass (fig. 25). The nuclei stain with increas- 
ing intensity. The tails of the sperm appear as stiff rods, finely 
attenuate, held rigidly at right angles to the head and gathered 
