THE GASTR^A THEORY AND ITS SUCCESSORS. 101 



According to this scheme the central cells are diges- 

 tive and ingestive ab initio. It seems strange, however, 

 that cells specializing themselves for a purely digestive 

 function should withdraw themselves from practically all 

 contact with the surrounding medium, the source of the 

 food supply, and there seem to be mechanical difficulties 

 in the way of the central cells obtaining particles of 

 food, larger than those which could be ingested by the 

 locomotor cells, and through minute pores many times 

 smaller than the ectodermal cells. Furthermore the 

 explanation of the formation of the mouth is not at all 

 in harmony with its ontogenetic development. 



The cells which migrate are well nourished and there- 

 fore in a suitable condition for reproduction. This is 

 actually their function in Volvox, and in Protospongia, 

 which Metschnikoff cites, the only observations we have 

 indicate that the cells which leave the periphery of the 

 colony become reproductive. May this not be the origi- 

 nal function of the migrating cells, the formation of the 

 parenchymella being regarded as the migration to the 

 centre of eutrophic cells capable of being reproductive } 

 It does not necessarily follow that all the cells which 

 migrate must become reproductive ; some of them may 

 become specialized along other lines ; but the idea is sim- 

 ply that primarily it was cells in a condition suitable for 

 reproduction that migrated, and when only a few cells 

 underwent this change of position they all became repro- 

 ductive, but when a large number migrated some might 

 become differentiated to subserve other purposes. 



It seems to me that what we find in the development 

 of such a sponge as Halisarca indicates the manner in 

 which the solid embryo becomes converted into a hollow 



