PORIFERA, CQELENTERATA, VERTEBRATA 79 
study of the development of the odgonia, states 
that the youngest recognizable odgonia lie in the 
mesoderm, and his figure (Fig. 27, B) shows that 
they may be distinguished from neighboring cells by 
certain characteristics, among which is the presence 
of a darkly staining inclusion. In the adult sponge 
the amebocytes from which the oégonia and sperma- 
togonia arise occur in the middle layer of all regions of 
the body, but, as pointed out by Korschelt and 
Heider (1902), the odgonia and spermatogonia may 
develop in only certain definite regions (Plakina 
monolopha), or in groups (Aphysilla violacea) which 
contain a more or less definite number of cells and 
occupy a similar position in each individual (Eu- 
spongia). Such an aggregation is the most primitive 
form of ovary. 
Some of the amebocytes of the sponge are un- 
doubtedly germ cells (tokocytes) and are able to 
develop into oégonia or spermatogonia, or to form 
aggregations (gemmules, “ artificial gemmules,”’ “ so- 
rites,” etc.) which can “regenerate’’ an entire sponge, 
but whether the amebocytes that produce odgonia 
and spermatogonia are the same as the reproductive 
cells of the gemmules, the regenerative cells of the 
“artificial gemmules,”’ and amebocytes which form 
the buds in Tethya is still uncertain. It seems 
probable that they are all alike potentially but 
develop differently because of the effects of different 
environmental factors. The distribution of ame- 
bocytes with reproductive powers throughout the 
entire sponge-body accounts for the great regenera- 
