$ iv.] The question of species. 29 



familiar to us through the researches of Brefeld, van Tieghem, 

 Koch, and Prazmowski are comparatively monomorphous ; they 

 make their appearance in the vegetative segments of their de- 

 velopment as a rule in the same forms as regards their shape, 

 growth and grouping. Others show a greater amount of 

 variation in these respects; they display the phenomena of 

 pleomorphy in varying degrees. Among the endosporous 

 Bacilli described above Bacillus Megaterium is a particularly 

 good example of monomorphy. A motile rod is developed from 

 the spore and gives rise as it grows to successive similar gene- 

 rations of rods, until these at length proceed to the formation of 

 fresh spores (Fig. i). 



Bacillus subtilis when growing normally in a fluid differs to 

 some extent from B. Megaterium; successive generations of 

 rods moving about in the fluid proceed from the germinating 

 spores, but the later generations which proceed from them re- 

 main united into long filaments and are without motion, being 

 grouped together and forming on the surface the Zoogloea- 

 membrane mentioned on page 12. In this state they then form 

 fresh spores. Here therefore we have a small amount of 

 pleomorphy, two, or reckoning the spores, three distinct forms, 

 and in a sequence also which is regularly repeated from one 

 spore-generation to another. Moreover the special conditions 

 of shape and size always remain the same within certain limits 

 of variation, for variations in the direction indicated certainly 

 occur in this case as they do everywhere in the organic world. 

 Stunted forms may also be met with. I have for instance re- 

 peatedly observed some of the rods in a group of Bacillus 

 Megaterium, in circumstances unfavourable to its nutrition, sepa- 

 rate into its cells which were themselves already short, and these 

 cells round themselves off and in this way represent what may 

 be termed Cocci. Other unusual forms also made their appear- 

 ance with the Cocci. There was scarcely any or no formation 

 of spores. Under improved food-conditions these stunted forms 

 reverted to the normal state. 



