PORIFERA, CCELENTERATA, VERTEBRATA 79 



study of the development of the oogonia, states 

 that the youngest recognizable oogonia 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 oogonia 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 oogonia and spermatogonia may 

 develop in only certain definite regions (Plakina 

 monolopha), or in groups (Aphysilla violaced) 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 oogonia or spermatogonia, or to form 

 aggregations (gemmules, " artificial gemmules," " so- 

 rites," etc.) which can "regenerate" an entire sponge, 

 but whether the amebocytes that produce oogonia 

 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- 



