62 



THE AMERICAN MONTHLY 



[April, 



having seen any solid particles within 

 the ectoderm, supposed that it was 

 nourished entirely by organic matter 

 in solution in the sea-water. It has 

 long been known that the ectoderm 

 of hydroid polyps protrudes pseudo- 

 podia which frequently anastomose 

 to form a kind of plasmodium, and it 

 occurred to Metschnikoft' that these 

 pseudopodia might have the function 

 of picking up food particles. This 

 was really observed to be the case 

 with the nematocalyces of Plumula- 

 rla {setacea.) When powdered car- 

 mine was suspended in the water 

 surrounding a Plumularia^ it was, 

 after some little time, seen in consid- 

 erable quantities within the substance 

 of the ectoderm of the nematocalyces. 

 Furthermore, colonies of Pluniularia 

 polyps, observed in a watch-glass, 

 will only live a short time after gather- 

 ing ; but only the polyp heads die, 

 while the coenosarc and the nemato- 

 calyces survive, which latter may be 

 seen eating up the dying hydranths. 

 Thus, after the polyp has retracted 

 its tentacles and become a mere 

 rounded mass, the free end of a ne- 

 matocalyx creeps into the theca and 

 gradually absorbs, by means of its 

 ectoderm, the whole contents of the 

 cup. These so-called nematocalyces 

 are therefore classed among organs 

 whose chief function is prophylactic ; 

 they eat up necrotic parts of the 

 colon}', and also continually explore 

 the organs in this vicinity, in order 

 to render harmless any injurious 

 bodies by devouring them. 



Ectodermal digestion has also been 

 observed to take place in Actinias^ 

 especially A. mesembryanthenium. 

 The solid particles contained within 

 the ectoderm are usually seen . sur- 

 rounded by a vacuole, thus indicating 

 the occurrence of some digestive pro- 

 cess. If a larval Actinia is taken 

 from its mother and placed in water 

 containing carmine in suspension, the 

 carmine granules are eaten by the ec- 

 toderm cells, being seized by means 

 of short pseudopodia, extended from 

 the free surface. After the develop- 



ment of the gastric pouches, however, 

 the number of foreign particles with- 

 in the ectoderm is much smaller. 

 As a further example of ectodermal 

 nourishment, Metschnikoff cites the 

 ovarian ova of those animals whose 

 generative cells are ectodermal ; for 

 example, those of Tubularia. and, 

 according to Kovotneft', of Hydra. 

 In the first named, Metschnikoff saw 

 the young ovoum eat and digest the 

 neighboring follicular cells, and Ko- 

 votneff says that during the winter 

 the young ectoderm cells of Hydra 

 devour the older ones. 



II. Intracellular Ingestion and 

 Digestion by Wanderittg Mesoderm 

 Cells. — While the taking up of nutri- 

 ment by ectoderm cells can only -be 

 observed in rare and exceptional 

 cases, nothing seems to be easier than 

 to find amoeboid cells of the meso- 

 derm which both ingest and absorb 

 food particles. Haeckel was the first 

 to observe that when a Tethys was in- 

 jected with indigo, the granules were 

 taken up by the blood corpuscles. 

 Later on he proved this occurrence 

 in the blood of various invertebrates, 

 and it wrfs this observation of Haeckel 

 which formed the starting point of so 

 many important researches in histol- 

 ogy and pathology. 



As objects for the study of this func- 

 tion in mesoderm cells, Metschnikoff' 

 chose the Auricularia of synapta, 

 and the Bipinnaria asterigera. At 

 the period of the metamorphosis of 

 these larval Echinoderms, which is, 

 as is well known, extremely compli- 

 cated, and associated with the loss of 

 many lai'val organs, these mesodermal 

 cells ingest the cellular debris of the 

 disappeai-ing organs and finally ab- 

 sorb them. Resorption-phenomena 

 can best be seen at two stages in the 

 life history of Auricularia. They 

 first occur at the assumption of the 

 so-called pupa stage, when a large 

 part of the longitudinal ring of cilia 

 is lost ; that is, is disintegrated and 

 devoured by the mesoderm. At this 

 time every amoeboid cell of the meso- 

 derm is generally loaded with enor- 



