IN THE PLASMODIA OF MYCETOZOA. 265 



Plasmodium can move freely, with its higher parts exposed to 

 the atmosphere. Bearing in mind that plasm, owing to its 

 peculiar molecular texture, is, in a high degree, both tenacious 

 and elastic, it will be easily understood that the plasmodium, 

 saturated by the moisture of the surface on which it rests, 

 becomes relaxed and motile underneath, while the parts exposed 

 to air remain more contracted. The contraction and relaxation 

 are, of course, affected by every alteration in the temperature 

 and humidity of the atmosphere. 



Another point to be borne in mind is the fact, already alluded 

 to, that the whole surface of the plasmodium, whether in contact 

 with air or water, is somewhat firmer than the interior plasm. 

 This surface plasm, called hyaloplasm, has no actual rigidity, 

 nor is it membraneous or impervious, or clearly separable from 

 the more fluid plasm beneath ; but it is less mobile than the 

 interior plasm, owing to conditions which govern surface tensions 

 at the areas of contact of different media. As seen through the 

 microscope, the hyaloplasm is a clear, transparent covering, 

 unequal in thickness, merging imperceptibly into the interior 

 plasm, and chiefly noticeable at the advancing margin of the 

 plasmodium. Its function is important. By its firmer con- 

 sistency, it preserves the integrity of the organism ; while at the 

 same time it yields to the impulses of the interior plasm 

 sufficiently to allow the plasmodium to move freely from place to 

 place. It is also probable that by its elasticity the hyaloplasm 

 reacts physically upon the more fluid interior plasm ; and this is 

 possibly what Haeckel referred to when he mentioned that 

 experiments had shown that counter-movements of Mycetozoa 

 are provoked by a peculiar form of pressure sensation, produced 

 by the flow of liquids ; but if so, it is not a very clear way of 

 stating the case. 



Reverting to the plasmodium of B. ulricularis, and surveying 

 its general features, we notice that, whatever its shape may be, 

 its substance is always distributed unequally. When feeding, it 

 aggregates into irregular masses ; but when foraging for food 

 these become smaller, because shorter or longer extensions are 

 protruded from them. These extensions, which at first have the 

 appearance of pseudopodia, become very attenuated, spreading 

 about in the form of a straggling network of larger and smaller 

 veins, with few or many angular or rounded meshes. The 



