Historical sketch on Immunity 517 



ectoderm and the entoderm appeared quite clear, and the former 

 might quite reasonably be regarded as the cutaneous investment of 

 primitive multicellular animals, whilst the latter might be regarded 

 as their organ of digestion. The discovery of intracellular digestion 

 in many of the lower animals led me to regard this phenomenon as 

 characteristic of those ancestral animals from which might be derived 

 all the known types of the animal kingdom (excepting, of course, 

 the Protozoa). The origin and the part played by the mesoderm 

 appeared the most obscure. Thus, certain embryologists supposed 

 that this layer corresponded to the reproductive organs of primitive 

 animals : others regarded it as the prototype of the organs of loco- 

 motion. My embryological and physiological studies on sponges led 

 me to the conclusion that the mesoderm must function in the hypo- 

 thetically primitive animals as a mass of digestive cells, in all points 

 similar to those of the entoderm. This hypothesis necessarily attracted 

 my attention to the power of seizing foreign corpuscles possessed by 

 the mesodermic cells. This fact has long been recognised. It was 

 known that the white corpuscles of the Vertebrata often contained 

 various kinds of cells, especially red and white blood corpuscles. It 

 was known, also, that the amoeboid cells were capable of ingesting 

 granules of coloured substances. When making an injection of 

 indigo into the vessels of Thetys, Haeckel 1 in 1858 was surprised to 

 find the blue granules inside the amoeboid blood corpuscles of this 

 beautiful gasteropod mollusk. This fact has since been confirmed by 

 many observers, and the capacity of the amoeboid cells to take up 

 foreign bodies became recognised as a general phenomenon. Never- 

 theless this phenomenon was not regarded as being analogous to 

 digestion. Thus Haeckel 2 himself, in his researches on the calcareous 

 sponges, advocated the view that the foreign bodies penetrated 

 into the interior of the viscous protoplasm in a purely passive 

 fashion. 



Observations that I made on sponges and on certain pelagic [511] 

 animals, transparent and of simple organisation, convinced me that 

 the presence of foreign corpuscles in the amoeboid cells of the meso- 

 derm must be attributed to an active ingestion by these cells which, 

 in every respect, might be compared to the phenomena of intra- 

 cellular digestion in the epithelial cells of the digestive canal of 

 many of the lower animals. In order to demonstrate this fact 



" Die Radiolarien," Berlin, 1862. 

 2 " Die Kalkschwamme," Berlin, 1872. 



