DIFFERENTIAL INHIBITION IN OBELIA 491 



elements se desagregent,' after which process the phagocytic 

 action commences. Most authors are also agreed that a process 

 of dedifTerentiation initiates the resorption of the tail in Anm-an 

 tadpoles, phagocytosis being secondary (cf. Naville, 1922). 



Phagocytosis here only occurs after resorption, i.e. after the 

 tissue elements have migrated from the tissues. It would appear 

 not to be a normal process in Hydroids, but to be a result of 



(a) the power of the endoderm cells to ingest solid particles, 



(b) the presence of abnormally situated cells which have migrated 

 out of the tissues in the neighbourhood of the endoderm cells. 

 Phagocytosis of this nature appears to occur both in the case 

 of emigrated endoderm cells of the zooid, and of normal endo- 

 derm cells in the walls of the stem. 



Eesorption (as a result of emigration of dedifferentiated cells) 

 may be regarded as the most primitive method of eliminating 

 tissues in Metazoa. Even at the outset it may be secondarily 

 accompanied by a low form of phagocytosis. Later the 

 function of phagocytosis is assigned to special cells, and the 

 dedifferentiating tissues are attacked by these at a much earlier 

 stage in the process. The limited extent of phagocytosis in low 

 forms is also to be seen in Planarians (Vandel, 1921). 



Eesorption is in the first instance a direct result of exposure to 

 unfavourable agencies, but may be utilized later as a method of 

 accomplishing normal processes of the life-history. This appears 

 to be the case in Echinoderm metamorphosis (Huxley, 1922). 



The stimulus in the case of Obelia is a certain concentration 

 of toxic products in the water. It is the same stimulus which 

 causes dedifferentiation in Clavellina and Perophora, and also 

 in Echinoderm larvae, Planarians, Sponges, and Protozoa 

 (Lund, 1917). 



Hunger is another stimulus which may cause dedifferentia- 

 tion ; and of course may act differentially. Dedifferentiation 

 caused by hunger has been found in Hydra (Schultz, 1906), 

 Echinoderm larvae (Runnstrom, 1917), &c. Starvation will 

 again act by interfering with general metabolic processes. As 

 an example of the differential action of hunger, it may be 

 mentioned that in starved tadpoles, localized dedifferentiation 



