184 



GENERAL PHYSIOLOGY 



cases are known in single cells. Thus A. Schneider, Brass, and 

 O. Zacharias ('85) have produced considerable changes of form 

 in spermatozoa, intestinal epithelium-cells, and Amoeba, by the 

 addition of various solutions to the medium. Unicellular organisms 

 in general, especially Infusoria and Rhizopoda, afford many favour- 

 able objects for the study of the changes that the body-form 

 experiences as the result of changes in the surrounding medium. 

 The following example 1 is very interesting; it shows that the 

 various forms of Amoeba, which are usually characterised by the 

 shape of the pseudopodia, ought not to be regarded as distinct 

 species in the systematic sense. Innumerable quantities of small 

 amoebae are frequently found in the bacterial scum upon the sur- 

 face of decomposing hay-infusions. When placed upon the slide, 

 these have an essentially spherical form (Fig. 66, a). Broad, lobate 

 pseudopodia begin gradually to be extended 

 in various directions, so that the form of 

 Amoeba proteus (princeps) (Fig. 66, &) is as- 

 sumed. The creeping soon takes on one 

 principal direction, the whole cell in a 

 certain sense representing a single, long 

 pseudopodium and assuming the form of 

 Amoeba Umax (Fig. 66, c). In this form the 

 amoebae creep about constantly, so long as 

 they are not disturbed. If the composition 

 of the medium be changed by making the 

 water very feebly alkaline by the addition of 

 potash solution, the following is observed. 

 The amoeba? first contract into balls, but 

 soon fine-pointed pseudopodia appear upon 

 their surface (Fig. 66, d). These become 



longer and longer, and finally assume the appearance of long, 

 pointed thorns. In the course of about 15 or 20 minutes the cells 

 assume the very characteristic shape of Amoeba radiosa (Fig. 66, ,/), 

 which is known by the systematists as a very well-defined 

 species ; they remain in this condition and show the very sluggish 

 movements of this species so long as the alkalinity of the medium 

 continues. If they are put again into their accustomed water, 

 their shape changes gradually to the usual Umax-form. Many 

 moulds, which can be accustomed to concentrated salt-solutions 

 when these contain sufficient food-stuffs for Mucor, behave similarly. 

 The hyphae, as a rule, become considerably finer and slenderer than 

 in the customary water. In many cases, however, changes in the 

 vital conditions affect, not directly the form of the individual, but 

 in a hidden manner the germ-plasm of the sexual cells, so that the 

 offspring assume forms different from those associated with earlier 

 conditions ; this, however, is rather to be considered under phyletic 



adaptation. 1 Of Verworn ('96, 4). 



FIG. 65. A. Branchipus stag- 

 nalis, fresh-water form ; 

 B. Artemia salina, salt- 

 water form of the same 

 crustacean. (From Sem- 

 per.) 



