BACTERIA AND THEIR ALLIES 165 



becoming visible in the midst of homogeneous protoplasm ; such 

 particles being invariably motionless but followed soon by the 

 development therefrom of definite Bacteria or their allies, recog- 

 nisable as such by their shapes and modes of collocation. 



These two points are, therefore, of great importance, and for the 

 purpose of interpretation it should always be borne in mind that 

 in cases of Infection by Bacteria and their allies we have to do 

 with adult organisms in a state of activity ; while in cases where 

 Heterogenesis may be presumed to be occurring we have invariably, 

 in the first place, to do with germs and motionless organisms, ' 



We must now see, in the first place, what cogent evidence can 

 be obtained in regard to the origin of microorganisms within the 

 tissues of animal organisms. 



It would be useless to multiply instances. I will, therefore, first 

 cite a single case in which the origin of Bacteria may be actually 

 watched within the body of a low animal organism, and then turn 

 to their mode of appearance within some of the tissue elements of 

 vertebrates. 



Evidence of a particularly convincing nature is to be obtained 

 from the examination of a little creature low in the scale of animal 

 life, namely, Cyclops quadricornis, one of the Entomostraca so com- 

 monly to be found in ponds. It may be seen from PI. xxiv. of 

 Baird's " Natural History of the British Entomostraca " " that the 

 four pairs of abdominal feet and also the tail are furnished with 

 a number of "plumose spines or setas." 



Examination of one of these organisms will show that within the 

 chitinous envelope of these slender spines, which taper away to 

 sharp points, there is nothing but structureless protoplasm to be 

 seen (Fig. i. A, x 700). If we take one of these little creatures, 

 put it in a drop of distilled water, on a glass slip with a fragment 

 of a No. 2 cover-glass on each side of it, and place over all a cover- 

 glass, it will be found that the animal is soon killed by the weight 

 of the latter though the fragments of glass prevent rupture of the 

 body. We may then place the microscope slip in a Petri dish 

 containing a thin stratum of water (so as to prevent evaporation 



' Of course by " germs " I mean here merely minute and undiiferentiated 

 stages of the organisms in question, produced by Heterogenesis, and not the 

 ordinary acceptation of the word, viz., a reproductive unit formed in an organism 

 of like kind. 



= ' Ray Society ' Publication, 1850. 



