The Action of Gravity upon Bacterium Zopfii. 307 



chiefly owing to the filaments slightly burrowing into the gelatine. 

 The filaments may be very long and imbraiiched, but when branching 

 does occur it appears to be brought about as in Gladothrix by the- 

 deflection of a segment to one side and subsequent growth. Soon after 

 they are formed the long slender rami exhibit a remarkable proneness 

 to twist and to segment, and then to pass into the zoogleaform or, less 

 commonly, into the skein phases. The twisting is very striking, and 

 some idea of it can be gathered from figs. 19, 25, 26, 27, 28, 29, 30. 

 In none of these photos is the spirilliform twist of single filaments 

 shown ; it is, however, exceedingly common ; the spiral is neverthe- 

 less seen in the zooglea phases, figs. 25, 26, 30. There are, therefore,, 

 twists in the horizontal plane, and spirilliform twists. Concerning 

 these twists it appears very remarkable that the majority appear to 

 turn inwards, that is, in the opposite direction to the hands of the 

 clock. To this point we will return when discussing the geotropism. 

 It will be gathered from the above and from the photos that the 

 segments may exhibit the "form phases " long ago laid stress upon 

 by Zopf and Lankester; we need not, therefore, dwell upon them, 

 The spiral is a striking feature in all gelatine preparations, but is 

 only slightly marked in agar, whilst we have not as yet observed it 

 in broth ; but, we may add, we have not examined a sufficient 

 number of these latter specimens. 



Skein Phases. The meaning which we wish to convey in this name 

 is set forth in figs. 21, 22, and 23. We lay no stress upon it, but tho 

 term is convenient, for it is the form which can, we think, always 

 be seen upon agar and stiff gelatine. It will be seen that it is quite 

 unlike the zoogleaform phase. The filaments composing it are not 

 held together by a common cementing substance, but are formed by 

 plaits taking place in the course of a filament, as seen in figs. 21 and 

 19. Fig. 23 from an agar culture will at once recall the medusiform 

 appearance of the anthrax growth. Its formation appears to be very 

 closely related to the stiffness of the medium upon which it is grow- 

 ing, and appears always to occur upon the surface of the medium ; the 

 skeins are usually moist, and to the eye present a glistening frosted 

 appearance. 



Zoogleaform Phases. This is the phase one commonly, sees upon 

 gelatine, and its striking appearance is well represented in figs-. 

 24 31. Zoogleas are said to result from either the aggregation of 

 dissociated elements or from the multiplication of segments which 

 remain where they are formed, being held together by a common 

 cementing substance. Of the former mode of formation we have no 

 experience, and we venture to think that it is not common. If one 

 turns to the descriptions of the higher bacteria, the impression is 

 gained that the zooglea masses have resulted from local multiplica- 

 tion ; that this is so in Bacterium Zopfii the photographs show, as 



