THE SPERMATOPHORE IN ARENICOLA 1015 
since the cells are growing at the same time they are dividing in 
the nutritive body fluid. Finally, the last generation of spermato- 
gonia is formed, measuring about 2.9u. They transform into the 
spermatocytes of the first order, with a diameter of 3.24. These 
divide to form the spermatids with a diameter of about 2u. In 
A. claparedii and A. grubii the spermatids are somewhat larger, 
measuring some 2.4y in diameter; in the other species the cells 
vary very little in size from the measurements given for A. cristata. 
Analogy to blastulae 
There is thus formed a hollow sphere of cells, reminding one of 
the blastula of the segmenting egg (fig. 23). This likeness is 
even more emphatic when one follows the history of a single 
spermatogonium when freed from the surface of the testis. It 
will be recalled that as well as the spherical masses of several 
dozen cells, smaller masses, even individual cells, are discharged 
into the coelomic fluid, so that one finds in this fluid all stages in 
the formation of the spermatophore from the one-celled condi- 
tion up to the completed structure. Naturally the resulting 
spermatophores vary very decidedly in size according as they 
have origin in a single discharged spermatogonium or in a coherent 
mass of such cells. Measurement of the mature spermmasses 
gives a variation, their diameters ranging from one to eight. 
The single spermatogonium, floating freely in the coelomic fluid, 
divides into two, then four, eight, sixteen cells, etc., simulating in 
general appearance the cleavage of an egg. Further discussion 
of this matter will be taken up below when the division of certain 
giant spermatogonia is considered (figs. 17-25). 
During the process of cell multiplication the forming spermato- 
phore is constantly increasing in diameter. Since the cells form 
only a single layer, the smaller they become the thinner the wall 
of the sphere is and hence the larger it may be with a given amount 
of protoplasmic material. This protoplasmic material must also 
increase in amount during the spermatogonial divisions by appro- 
priation of nutritive material from the coelomic fluid, for the 
spheres increase in diameter more than the mere thinning of their 
