FROM ONE CELL TO MANY CELLS 71 



or form special cells which reproduce the group in another way; but none 

 of the 4 small cells can do this. In another species (Fig. 56) the cells are 

 more numerous, and the sterile and reproductive cells are more nearly 

 equal in number; but they are again of two sizes, the smaller ones being 

 sterile. Volvox (Fig. 57) is another, though much larger, form in which 

 there are sterile and reproductive cells; but here the sterile cells greatly 

 outnumber the reproductive. The two Pleodorinas and Volvox, taken in 

 the order in which they are used here, show an increasing number of the 

 sterile Cells. 



The existence of such forms as these suggests that the earliest differ- 

 entiation between the cells of a colony, on its way to becoming a met- 

 azoon, was the loss of reproductive powers by some of the cells. The 

 group of sterile cells in these organisms corresponds to the soma, or body, 

 as contrasted with the germ cells, or reproductive cells, of the metazoa. 



Further Differentiation. — In the organisms just studied, all the sterile 

 cells are alike in structure and function, except in Volvox, in which the 

 cells on the front side, as the organism swims, differ 

 slightly in color and the size of certain of their structures 

 from those on the rear side. This is quite at variance with 

 the higher metazoa, in which the cells of the soma are of 

 very many markedly different kinds. There is no way of 

 knowing which of the manj^ types of somatic cells originated 

 earliest; hence no clue as to what kind of modern animal Fig. 58. — 



we should look for to illustrate that step. The best we ^^ '^senemi 

 can do, if we are to pursue this plan of choosing present- form of body, 

 day representatives, is to select some animal in which the ^' 

 differentiations among the somatic cells are not too numerous. A 

 suitable form is the fresh-water Hydra, in which half a dozen kinds of 

 somatic cells are found. A brief description of the body as a whole must 

 precede the study of these cells. 



The form of Hydra is essentially cylindrical (Fig. 58) when extended 

 and more or less globular when contracted. Ordinarily the body is 

 attached 'by one end, the foot, to a solid object. At the tip of the free 

 end of the body the mouth is located. Near the mouth is a circlet of 

 long contractile tentacles which have arisen from the body by an out- 

 pushing of the body wall. By means of the tentacles Hydra captures and 

 thrusts into its mouth minute aquatic animals. The conical eminence 

 between the mouth and the tentacles is the hypostome. 



The body of Hydra is hollow (Fig. 59), the interior space being a 

 digestive cavity. Its wall is composed of two layers of cells, the outer 

 known as the ectoderm, the inner as the endoderm. The endoderm cells 

 are all essentially alike, being tall and slender and bearing flagella. Their 

 function is the digestion of food. The ectoderm has differentiated into 



