166 HBTEROGENETIC ORIGIN OF 



from beneath the cover-glass) and fixing upon one of the tail setae 

 (these being larger than those on the abdominal feet), we may 

 examine it from time to time. What may be observed is this. 



After an interval of two or three days (the duration depending 

 upon the temperature of the air at the time) we may see, under 

 a high power of our microscope, scarcely visible motionless specks 

 gradually appear in increasing numbers in the midst of the struc- 

 tureless protoplasm ; and, still later, we may see some of these 

 specks growing into Bacteria, as in Fig. i, B, which is a represen- 

 tation of A after four days. At last the whole interior of the spine 

 becomes filled with distinct Bacteria, as may be seen in C, which 

 is from a photograph of the same spine on the sixth day — the 

 temperature during these days varying from 70-75° F. Later still, 

 all the Bacteria, previously motionless, begin to show active 

 swarming movements. 



In such a case it is clear we have to do with no process of 

 infection from without, but with a de novo origin of Bacteria from 

 the protoplasmic contents of the spines or setae. The fact that 

 they appear in these situations as mere separate, motionless specks, 

 and gradually take on the forms of Bacteria (also motionless at 

 first), is, as I have previously indicated, just what we might expect 

 if they had actually taken origin in the places where they appear. 

 On the other hand, such a mode of appearance is totally opposed 

 to what might be expected, if the microorganisms had obtained an 

 entry from without, through the tough chitinous envelope of the 

 spines. It will be observed also that the apparent origin and mode 

 of appearance of Bacteria here is precisely similar to what is to be 

 seen, when a film of a suitable organic fluid is watched under the 

 microscope, and Bacteria gradually make their appearance therein 

 (see p. 52). 



I pass now to what may be regarded as another absolute proof 

 of the heterogenetic origin of Bacteria, as convincing as that which 

 will be subsequently shown to occur within the closed cells of 

 certain vegetables. 



I have already pointed out that in many parts of the bodies of 

 man, and of higher animals generally, microorganisms are known 

 to exist in abundance. This is the case, for instance, throughout 

 the whole length of the alimentary tract, and throughout a con- 

 siderable extent of the mucous membranes of the respiratory tract. 

 It is clear, also, that some of the microorganisms may be taken up 

 from these mucous membranes by lymphatics, and if they pass the 



