De Zouche. — Bacteria and their Relation to Disease. .51 



mesodenn, and in it are found the blood-vessels, muscles, 

 internal skeleton, and the excretory organs. What \ve have 

 now to- note is that to the mesodenn belongs the vascular 

 system. 



The mode in which the Amoeba envelopes and disintegrates 

 its food gives an example of intracellular digestion. This is 

 the mode of nutrition in the Protozoa, whether singly or aggre- 

 gated together in colonies. In Plumularia the ectoderm cells 

 are able to take up foreign particles, and to eat up dying or 

 dead portions of the annual or colony to which they belong. 

 In the larvai of the Actinias also the ectoderm cells have the 

 faculty of ingesting solid food. But already a differentiation 

 of function begins to be observed, and at a later stage of 

 development the number of foreign particles in the ectodei'm 

 cells becomes much smaller. The function of intracellular 

 digestion, we shall find, becomes the hereditary property of 

 the amoeboid cells of the mesoderm, and in these we should 

 expect to find it in vertebrate animals if it existed in them. 

 These cells have the property of being able to wander about in 

 the body of the animal (as observed hy Metschnikoff in Phyl- 

 lirhoe, a transparent mollusc). They devour all dead or dying 

 matter. In the case of large masses to be eaten, or foreign 

 particles to be removed, the amoeboid cells join their forces, 

 becoming fused together in cell-masses termed plasmodia, 

 which are equivalent to giant cells ; or the individuals remain 

 distinct, but swarm together in large numbers to the attack. 

 They have a power of selection, eating some objects presented 

 to them and refusing others. Bacteria are attacked and in- 

 gested by them, but occasionally the wandering amoeboid cells 

 are killed by the bacteria. These cells thus guard the body of 

 the animal to the best of their power against harmful sub- 

 stances, while they, in destroying foreign or dead matters, are 

 simply feeding, as I surmise, primarily for their own indi- 

 vidual nutrition as independent cell-animals, secondarily for 

 the nutrition of the colony of cells, or animal, of which they 

 form a part. Metschnikoff calls them phagocytes, or eating- 

 cells. Thus, in the lowest animal-forms all the cells are amoe- 

 boid, and all are phagocytes ; but as we ascend in the scale 

 we find the cells becoming diiiferentiated, some losing the 

 digestive power, while those of the mesoderm retain it. 



Following up this line of inquiry, M. Metschnikoff lias 

 ascertained the existence of phagocytes in the mesoderm of 

 vertebrates. These are the white blood-corpuscles. The 

 power of these leucocytes to wander outside of the blood- 

 vessels has already been mentioned. 



When the tails of tadpoles are undergoing absorption, 

 amoeboid cells in large numbers may be seen surrounding the 

 muscles and nerve-fibres, which they gradually devour, and 



