TISSUE FORMATION 305 



is balanced, as far as these cells are concerned, cannot be produced, since 

 they need a protective colloid in the form of protein, as has already been 

 stated in the case of Limulus. However, subsequent investigators (Brian, 

 Ries) attribute the tissue formation during the process of budding, not to 

 these amoeboid cells, but to special cells which resemble more closely lympho- 

 cytes and which have a tendency to divide mitotically. Ries assumes that the 

 packages of amoeboid cells which are seen, serve merely as foodstuffs during 

 the process of tissue formation ; but even if this view should be correct, still, 

 the amoeboid cells of Clavelina do produce tissue-like formations during or 

 preceding the process of budding and migrate towards the regions where 

 active tissue formation occurs, and in this respect they resemble in their 

 mode of reaction the amoebocytes of Limulus and the archaeocytes of 

 sponges under injurious conditions. 



There is some indication that also in other instances the blastema from 

 which regenerative processes proceed, takes its origin from cells migrating 

 to a wound from distant parts of the organism. Observations of this kind 

 have been made by Balinsky and Hellmich, and we have referred to them in 

 a previous chapter. There is, however, some doubt at present as to whether 

 we have to deal in these processes with the migration of more or less un- 

 differentiated mesenchymatous cells possessing great developmental poten- 

 tialities, or with the migration of already more or less differentiated cells 

 giving rise to the new tissue. If the migration of undifferentiated wandering 

 cells should actually play so great a part in regenerative processes as is as- 

 sumed by Hellmich, it is quite probable that here, also, agglutination and 

 possibly coalescence of these cells precede tissue formation. 



5. Tissue formation which takes place during embryonal life begins with 

 the segmentation of fertilized or parthenogenetically developing ova; but in 

 this case, underlying the union of the cells is a more complicated mechanism. 

 This depends, above all, on the presence of membranes surrounding the 

 ovum and the early embryo, and furthermore, on certain special structures 

 which connect the individual segments. However, the methods which are 

 successful in accomplishing the union of different ova or blastomeres, or in 

 separating normally united blastomeres from each other — both processes 

 being influenced by changes in alkalinity, in Ca content, and in the tempera- 

 ture of the surrounding medium — indicate that also in these cases we may 

 primarily have to deal with agglutination processes due to changes in the 

 consistency of the ectoplasm of ova or blastomeres. These primary changes 

 may then be secondarily followed either by coalescence or by fargoing cell 

 and tissue differentiations. It may be assumed, therefore, that also in the first 

 stages of the formation of multicellular embryos, agglutination processes, 

 not unlike those which occur between amoebocytes of Limulus, may play a 

 significant role. 



With this conclusion harmonize also the experiments relating to the 

 agglutination and coalescence of ova and blastomeres in various classes of 

 animals, to which we have referred in a preceding chapter. The organismal 

 differentials or their precursors were found to be an essential factor in de- 



