184 HETEROGENETIC CHANGES IN 



than usually abundant, found on a hay infusion on the fourth day. 

 They are rendered more distinct owing to the ground-work of the 

 pellicle having been stained by a weak eosin solution. B ( x 500) 

 shows a portion of such a mass highly magnified. 



When we attempt to remove a small portion of the early and 

 almost invisible scum on the tip of a sterilised scalpel, we may 

 often see that its point will very perceptibly depress the surface 

 before it breaks through the membrane. This is especially the 

 case when infusions of a greater depth than those I now recom- 

 mend are employed, and then, also, each day that passes adds to 

 the thickness of the pellicle, till after seven or eight days have 

 elapsed it may have been converted into a comparatively thick 

 pulpy layer, owing to continued accretions of fresh Bacteria from 

 below. After this period, or even before, the superficial layer ©f 

 the pellicle may gradually become more and more brown, while 

 rounded, or branched and elongated masses of zooglcea project 

 from its under surface. Ultimately, after three or four weeks, the 

 pellicle, if left, may break away and in part sink to the bottom of 

 the vessel. 



Such are the changes to be met with during the formation and 

 growth of a pellicle on the surface of a hay infusion made and 

 exposed in the manner I have described. One of the most notable 

 points in connection with it is the fact, that from a very early period 

 the Bacteria, which thus aggregate into a scum at the surface of the 

 fluid, where they are freely exposed to air, excrete a transparent 

 jelly-like or glcsal substance by means of which the constituent 

 units of this scum become blended into a thin elastic membranous 

 layer. 



As may be supposed, when we examine some of the turbid fluid, 

 or the thin scum that first appears, Bacteria of several different 

 kinds are pretty constantly met with ; but, for the most part, with 

 hay infusions, it is Bacilli which largely predominate, such as may 

 be seen in Fig. 24, A ( X 500). Toruloid corpuscles are decidedly 

 rare, and no Monads^ Amcebae, or Ciliates are ever to be seen at 

 this early stage. The mode and times of their appearance will be 

 presently dealt with. In the first place, however, I would 

 emphasise the fact that the researches which I am about to detail 

 have no pretence to be conducted in ways that are proper and 

 usual in the great bulk of bacteriological inquiries. We start here, 

 confessedly, with a mixed association of Bacteria, tending to 

 aggregate in a promiscuous manner on the surface of the fluid, 



