IMMUNITY AGAINST MICROBES. 101 



devoured by the mesoderm — and, secondly, during the meta- 

 morphosis of the pupa into a young Synepta, when mesodermic 

 cells become active again, collect beneath the ciliated rings, 

 and eat up the products of disintegration. Every amoeboid 

 cell during both these stages is generally loaded with an 

 enormous mass of debris-granules contained in the interior of 

 clear vacuoles. 



In Asterides also, whole segments of larvse are absorbed by 

 mesodermic cells during the metamorphosis. This process, in 

 fact, appears to be the rule in all Echinoderms. 



Mesodermic cells were supposed by Ganin^ to play an active 

 part in the histolysis of muscular fibres during the metamor- 

 phosis of flies. More lately, Kowalewsky^ showed that Ganin's 

 supposition was correct, for he proved that large numbers of 

 wandering cells penetrate between the muscular fibres, take 

 into their interior and digest the latter. 



Mesodermic cells are active not only during the period of 

 metamorphosis, but under many other circumstances. Schneider^ 

 noticed that the resorption of the genital organs of Hirudo is 

 carried out by amoeboid cells greatly resembling blood- 

 corpuscles, and Metschnikofi" described the same phenomenon 

 as taking place in the Aureliaaurita.* The developing nemer- 

 tine in a pilidium also, if the latter is left for some length of 

 time in a glass vessel, atrophies and is devoured by amoeboid 

 mesodermic cells. Some of the mesodermic cells of transparent 

 salt-water animals taken fresh from the sea are filled with 

 foreign matter, and Metschnikoff assumes that such foreign 

 bodies gain an entrance by piercing through the body- wall, 

 being ultimately swallowed by amoeboid cells. Particles of 

 carmine suspended in water not only penetrate into the cells 

 of the entoderm, but into those of the mesoderm also. 



The following experiments throw much light on the pro- 

 cess of intra-cellular digestion by mesodermic cells. When 



' Ganin, ' Beitrage, &c./ 1876, p. 40. 



^ Kowalewsky, ' Zeitschr. f. wiss. Zool.,' vol. xlv, 1887. 



3 Schneider, ' Zool. Anzeiger,' 1880, p. 19. 



* Metschnikoff, loo. cit., p. 89. 



