190 LECTURES ON BIOLOGY 



On starch-containing substrata moist bread, potatoes, etc. 

 appear sometimes little red spots which look like drops of 

 blood. This strange phenomenon gave rise to the legend of 

 the ' bleeding host/ and was frequently employed for the pur- 

 poses of the Church. The cause of the ' bleeding ' of the bread 

 is a minute organism, the so-called miracle-monad (Micrococcus 

 prodigiosus) , whose colonies are distinguished by a bright blood- 

 red colour. But if we take this little bacterium from its ordinary 

 nutrient medium and place it in an alkaline agar-agar solution 

 its red colour becomes less and less distinct and finally dis- 

 appears altogether. The vitality, however, of the micrococcus 

 is by this experiment in nowise injured, for if we return it 

 after a short time to its normal condition the red colour 

 reappears. But if we cultivate it on agar-agar for a considerable 

 time it grows afterwards white on bread and potatoes, and will 

 regain its normal colour only after many generations. We see 

 here quite clearly that changed conditions do produce a change 

 in the organisms, and that these are more permanent the longer 

 the new conditions have been in operation. 



We have already seen that irritability is one of the funda- 

 mental properties of the protoplasm. We saw, for instance, 

 that the little Amoeba Umax may be influenced both in its 

 vital functions and in its shape by mere changes of temperature. 

 If the temperature was reduced the animalcule became rigid and 

 assumed the shape of a sphere ; if the temperature was increased 

 it revived, unrolled itself, and crawled about, only at a further 

 increase in the temperature, once more to lapse into heat coma 

 and contract into a sphere. Still more interesting and im- 

 portant are those changes which we are able to produce in the 

 same animalcule by chemical influences. It is known that the 

 form of the pseudopodia in the various species of Amoebae is 

 the principal means of distinguishing between the different 

 species. Thus Amoeba Umax forms usually only one long 

 pseudopodium. A. proteus is distinguished by numerous shape- 

 less pseudopodia, and in A. radiosa, finally, the pseudopodia 

 radiate from the body in all directions in the form of long spines. 

 The last mentioned species, in particular, with its peculiar con- 

 struction, could not be mistaken for any other Amoeba. But let 

 us now observe A. Umax under the microscope. At first the 



