1891.] Wandering Cells in Animal Bodies. 513 
functions supposed to be possessed by amceboid cells in higher 
animals. 
In the embryo of Echinoderms, such as the starfish, there is a 
stage in which the organism consists of a single layer of cells in 
the form of a sphere, called a blastosphere or blastula, and its 
interior is filled with a jelly-like mass. Certain of these cells in 
the layer work out from between their neighbors by ameeboid 
movement, and come to lie free in this jelly (Fig. 1), where 
they move about. This is a fact of morphological interest, as 
these cells eventually form one of the primary layers—the germ- 
layers—of the embryo, which, by 
a definite development, form cer- 
tain organs of the adult. As the 
starfish blastula becomes older, it 
reaches a stage in which certain of 
its tissues break down, and these 
are not to pass into the body of 
the adult animal. Metschnikoff 
has observed that broken-down 
particles were taken up by the 
moving cells, as the Amceba takes , 
its food, and also found that they Fig.1 
were digested by the cells. Now Ue ok ae teen 
these particles, if allowed to re- to swim by means of cilia, showing amoe- 
main, would have been injurious °° °S- 
to the animal, so their assimilation by the amceboid cells was of 
great use to the individual. It was because of this peculiar 
function that the wandering cells were called phagocytes, or 
eating cells, and they are very generally known by this term. 
In order to establish a theory, now generally accepted, of the 
descent of the Metazoa, or many-celled animals, from the Protozoa, 
or single-celled forms, Metschnikoff sought for and proved the 
existence of this intracellular digestion in certain amceboid cells of 
sponges. Following out these facts, obtained in purely morpho- 
logical research, he was enabled to lay the foundation for certain 
views which are of the utmost practical interest, as we shall see, 
