1891.] MICEOSCOPIOAL JOURNAL. 233 



tissue a mass of cells. Let us now take one cell and compare it with 

 amoeba to see if it has the various protoplasmic properties, or some of 

 them. 



The Dermal Connective Tissue Cell. — If we could isolate a cell 

 of the dermis and observe it as we can do amoeba, we should know 

 much which we now hardly more than conjecture ; still, biologists have 

 so much confidence in their conclusions upon the matter that they often 

 state them as veritable matters of observation, and they are right in this, 

 for the sum of evidence in their favor justifies it. Like amoeba the 

 dermal cell consists of protoplasm in which lies a nucleus ; unlike amoeba 

 it lacks contractile vacuole, food vacuoes, and the power of motion by 

 means of pseudopodia. The dermal cell does not react quickly, so that 

 we cannot observe its power of motion and sensibility if it have any, 

 as we can do in amoeba, and its food is in solution so that we cannot 

 see it taken in as we can see in amboea. And yet there is evidence 

 which goes a considerable distance toward proving that the dermal cell 

 has all these powers. Let us examine the points of this evidence in 

 order. We noticed above that the frog's body is dark above and light 

 below ; a very careful examination of the case will show that the dark 

 color grows darker if the animal is kept in the dark, and lighter when 

 brought out into the light, and the study of closely-allied animals, for 

 example, the tree toad, will show some remarkable facts, for in these 

 not only the shade of color of the dermis is some way regulated to match 

 the surroundings of the creature, but to a certain extent also the tint 

 whereby they so closely imitate the color of their environment as to es- 

 cape observation. If we look closely into these facts we shall find 

 them instructive examples of cell-work. First, the production of 

 color. The blood contains none of the pigment ; it is a chemical ma- 

 terial made by the dermal cell out of materials derived from the blood ; 

 this form of metabolism is commonly called secretion ; these cells manu- 

 facture and retain their secretion. 



Not all dermal cells secrete pigment, and not all secrete the same 

 kinds. The failure to secrete accounts for the white color of the ventral 

 surface and the peculiar form of material secreted by difierent sets of 

 cells gives rise to the variously-colored markings on the frog's back. In 

 all of this we have a view of the case as involving the function of meta- 

 bolism variously in different cases with the ordinary color of the frog 

 as a result. In the second place, then, can we find any sign of motor 

 or sensory function from the dermal cell ? The fact of change of color 

 in response to the sru'roundings so well marked in the tree-toad, but 

 not wholly absent in the frog, can only be understood as a response on 

 the part of dermal cells to an affection from without themselves, and 

 this is by definition sensibility or irritability, a fundamental power of 

 protoplasm (see p. 127). The fact that the response is not very quick, 

 or the change of -color very great, does not alter the argument, and we 

 are justified in regarding this fact in the color of the frog as evidence of 

 the possession on the part of the pigment cell of the power of irritabil- 

 ity. The third fundamental power, the power of motility, is less ob- 

 viously present in these cells, but the exercise of this function is believed 

 to explain the mode of color change, for it is thought that by contrac- 

 tion or expansion on the various cells the pigment substances contained 

 therein are displayed or concealed, or altered in amount, and so the ob- 



