TISSUE FORMATION 303 



observed in free-living protozoa than to those observed in the amoebocytes, 

 which latter are adapted to a protein-containing environment, while the pro- 

 tozoa and sponges are adapted essentially to a medium which consists of a 

 mixture of salts. We may therefore regard the experimental amoebocyte tissue 

 as representing the most primitive and rudimentary type of tissue, and the 

 sponges as the next higher type, in which a further differentiation of the 

 component cells and their power to proliferate are added to the primitive mode 

 of agglutination and tissue formation. 



More recently the agglutinated sponge cells have been studied by Faure- 

 Fremiet in tissue culture in a similar manner to the amoebocyte tissue, and 

 he has shown that the archaeocytes behave, here, in about the same way as 

 the amoebocytes ; they move out of the peripheral piece of tissue in a centrifu- 

 gal direction and flatten out. It seems, also, according to this investigator, that 

 a further development of such a tissue culture into a typical sponge takes 

 place only if the archaeocyte tissue has become agglutinated to the surface 

 on which it has been placed. In both cases the processes leading to agglutina- 

 tion depend on changes in the ectoplasmic layer, which make it sticky, prob- 

 ably as a result of the taking-up of a certain amount of fluid by the stimulated 

 cell. In addition to the archaeocytes, the collencytes and the choanocytes take 

 part in the formation of the complete sponge, while the other structures are 

 produced through differentiation of these primary cells. It seems that the 

 excretory canals are the central organ around which the other structures are 

 built up. 



If the amoeboid cells of two different species of sponges, such as Micro- 

 ciona and Ciona, are mixed, two types of reaction may be noted: (a) When 

 separate archaeocytes of Microciona and Ciona come into contact the outer 

 hyaline layers of the protoplasm of the cells belonging to the different species 

 fail to coalesce, the cells of each species remaining separate and forming 

 aggregates of their own kind. Such a segregation is evidently caused by 

 differences in the physical properties of the outer protoplasmic layers of the 

 cells of these two species, and possibly, as in the case of pseudopods of certain 

 protozoa, by specific changes which take place in the consistency of the 

 protoplasm when cells possessing different species characteristics meet. There 

 may also be involved in this effect of foreign cells, either sessile contact 

 substances or substances secreted by these cells, or substances liberated from 

 the cells when they are injured in the preparation of the suspension. We may 

 have, in this case, to deal chiefly with the action of contact substances, which 

 lead to separation of cells if they are heterogenous ; in addition there may be, 

 as stated, direct physical differences in the cell membranes, which prevent 

 agglutination and normal tissue formation. In this connection it is of interest 

 to note that, according to Galtsoff and Pertzoff, the cells of Ciona and Micro- 

 ciona differ also in the pH of their cell content, (b) But there may take place 

 a second type of interaction between cells of different species. As a result of 

 the unfavorable effect of substances extracted from a suspension of heterog- 

 enous sponge cells, the archaeocytes are injured; rapid cytolysis take place 

 and the outflowing cytoplasm of the degenerating cells agglutinates to form 



