728 THE POPULAR SCIENCE MONTHLY. 



colored food, which will then be seen to have accumulated in the inte- 

 rior of their soft, transparent protoplasm ; and in some cases the color- 

 less blood-corpuscles have actually been seen to devour their more 

 diminutive companions, the red ones. 



Again, there are certain cells filled with peculiar colored matters, 

 and called pigment-cells, which are especially abundant, as constituents 

 of the skin, in fishes, frogs, and other low vertebrate, as well as many 

 invertebrate, animals. Under certain stimuli, such as that of light, or 

 of emotion, these pigment-cells change their form, protrude or retract 

 pseudopodial prolongations of their protoplasm, and assume the form 

 of stars or of irregularly lobed figures, or again draw themselves to- 

 gether into little globular masses. To this change of form in the pig- 

 ment-cell the rapid change of color, so frequently noticed in the ani- 

 mals provided with them, is to be attributed. 



The animal egg, which in its young state forms an element in the 

 structure of the parent organism, possesses in the relations now under 

 consideration a peculiar interest. The egg is a true cell, consisting 

 essentially of a lump of protoplasm inclosing a nucleus, and having a 

 nucleolus included in the interior of the nucleus. While still very 

 young it has no constant form, and is perpetually changing its shape. 

 Indeed, it is often impossible to distinguish it from the Amceba ; and 

 it may, like an Amoeba, wander from place to place by the aid of its 

 pseudopodial projections. I have shown elsewhere * that the primitive 

 egg of the remarkable hydroid Myriothela manifests amoeboid mo- 

 tions ; while Haeckel has shown f that in the sponges certain Amoeba- 

 like organisms, which are seen wandering about in the various canals 

 and cavities of their bodies, and had been until lately regarded as 

 parasites which had gained access from without, are really the eggs of 

 the sponge ; and a similar amoeboid condition is presented by the very 

 young eggs of even the highest animals. 



Again, Reichenbach has proved J that during the development of 

 the crawfish the cells of the embryo throw out pseudopodia by which, 

 exactly as in an Amoeba, the yolk-spheres, which serve as nutriment 

 for the embryo, are surrounded and ingulfed in the protoplasm of the 

 cells. 



I had shown some years ago that in Myriothela pseudopodial 

 processes are being constantly projected from the walls of the aliment- 

 ary canal into its cavity. They appear as direct extensions of a layer 

 of clear, soft, homogeneous protoplasm, which lies over the surface 

 of the naked cells lining the cavity, and which I now regard as the 



* " On the Structure and Development of Myriothela" " Philosophical Transactions," 

 vol. clxv., 1875, p. 552. 



f " Jenaische Zeitschrift," 1871. 



% " Die Embryonanlage und erste Entwickelung des Flusskrebse," " Zeitschrift fiir 

 wissenschaftliche Zoologie," 1877. 



Loc. cit. 



