304 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



a floccular material which likewise gradually becomes dissolved. There is, 

 however, no perfect correspondence between the phylogenetic relationship 

 of the two species and the way in which these heterogenous cells act on each 

 other; furthermore, the results obtained with reciprocal combinations may 

 vary. Whether the cytolytic substances involved in this process differ only in 

 quantity, or also in kind, from the contact substances mentioned above is 

 uncertain. It may be added here that in arthropods we have observed that 

 a precipitation takes place if the sera belonging to different species are mixed 

 with each other; this may be a related phenomenon to the cytolytic effect of 

 heterogenous substances seen in sponges. 



4. A still higher type of tissue formation has been described by Spek in 

 the tunicate Clavelina. Within this organism amoebocytes are found carrying 

 special cell inclusions and wandering to places where, owing to the presence 

 of a wound, regenerative or reduction processes occur, such as are associated 

 with bud formation. These amoebocytes migrate in great numbers and either 

 go to the area of new-growth or accumulate in the body cavity. Here they 

 agglutinate to form clumps or masses, arranging themselves in a tissue-like 

 manner, and according to Spek, subsequently giving rise to the formation of 

 the new tissues and organs. Under normal conditions when instead of other 

 amoebocytes they meet cells of a different kind, or if they migrate through 

 other tissue layers, they do not agglutinate with one another nor do they 

 agglutinate with the other kind of cells. But as soon as their environment 

 becomes abnormal, as for instance, near a wound, or when during the re- 

 duction processes in the animal they are exposed to conditions under which 

 abnormal products of disintegration act upon them, or when the preformed 

 tissues in these tunicates are unable to undertake the necessary regenerative 

 functions, then these cells become sticky and agglutination occurs. Preceding 

 the formation of clumps under such abnormal conditions, the amoebocytes 

 migrate in masses to areas which presumably have undergone pathological 

 changes, either to the aboral pole in dying animals or into the body cavity 

 prior to the formation of winter buds. In case of regeneration of special 

 organs they may first form epithelium-like surfaces, a process which likewise 

 presupposes agglutination. Smaller groups may then agglutinate with one 

 another, so that larger or sausage-like masses result, but the agglutination 

 processes are always preceded by active amoeboid movement, and this is an 

 oriented one, directed apparently by substances produced in regions where 

 pathological processes take place. These movements and agglutination proc- 

 esses are followed by organ formation. 



Thus we note here a close parallelism to the reaction of amoebocytes of 

 Limulus, where likewise abnormal environmental factors cause changes in 

 the surface layer of the cells leading to agglutination and formation of tissue- 

 like layers, but in Clavelina, as well as in sponges, these primary processes 

 are followed by the development of differentiated tissues and organs. There 

 is an additional point of similarity between amoebocytes of Limulus and the 

 amoebocytes of Clavelina ; for both of these types of cells sea-water or solu- 

 tions of inorganic constituents, as such, are toxic, and a salt solution which 



