LIFE CYCLE. 287 



cautions to exclude contamination by alien germs, either cutane- 

 ously, or, more commonly, subcutaneously. The cutaneous 

 inoculation is usually carried on upon places from which, like the 

 ear, it cannot be licked off by the animal. In subcutaneous 

 inoculation the skin, raised with forceps, has either a prick or an 

 injection made. With the guinea pig the inner side of the upper 

 part of the thigh, with the mouse the root of the tail is usually 

 selected for the operation. 



Life Histories. In nearly the whole of our microscopical 

 studies thus far, extending up to Chapter XX. of this work, we 

 have concerned ourselves only with the morphological character- 

 istics of vegetable organisms,] proceeding from the more complex 

 to those of extremely simple structure. In the group of the 

 bacteria, however, we have departed from this line of study. 

 Their morphological characteristics are so simple that in becoming 

 acquainted with them we have at the same time come face to face 

 with another set of phenomena, have realised that each of these 

 organisms has a life history or life cycle, In that life history 

 or life cycle we at least realise the existence of two characteris- 

 tics : that the organism, essentially unicellular, can nourish 

 itself, grow, and the cell which constitutes it undergo segmenta- 

 tion, so that each organism gives rise to two daughter organisms, 

 each of which is, practically, half of its parent ; we have, that is, 

 a method of multiplication of organisms of the most simple kind, 

 which we have called " fission '' ; that, on the other hand, under 

 certain circumstances many, at least, of these organisms can 

 undergo a sequence of changes in which the active life of the 

 individual is suspended, and its living substance is aggregated 

 into a body, the spore, or resting spore, which is capable of 

 passing into a period of rest, in which it can be dried and often 

 exposed to great extremes of temperature with impunity ; but 

 from which state of rest it can be aroused by suitable conditions, 

 and can then re-enter its ordinary phase of active, self-nourishing, 

 growing and multiplying life. It goes as a matter of course that 

 its nourishment is effected by means of the food substances con- 

 tained in the material in or upon which it was grow r n, whether 

 liquid or solid, and that these nutrient substances it manipulates 

 in such a way as to extract from them the constituents of its 

 own food. In this process it does not necessarily use up all the 

 chemical bodies of which the nutrient substratum is composed, 



