410 NATURAL SCIENCE [December 



Siebold, Dippel, Borscow, and particularly Mereschkowsky, regarded 

 the cell movement as the result of an osmotic phenomenon, supposing 

 water to be imbibed at the anterior end of the diatom and expelled 

 with greater force at the hinder extremity, the recoil serving to 

 propel the cell onwards. 



Biitschli and Lauterborn worked together at the subject, and 

 the former published a preliminary account of their researches, which 

 was adversely criticised by 0. Mliller. A reply from Lauterborn 

 drew a further communication from Miiller, and this is dealt with 

 in the work under consideration, where the Btitschli-Lauterborn 

 observations and inferences are given at length, with a detailed 

 criticism of Muller's objections and theory. The observations de- 

 scribed and illustrated by Lauterborn first claim attention, and will 

 be best given as nearly as possible in his own words. 



When large examples of Pinmdaria are brought into a concen- 

 trated emulsion of Indian ink, 1 the majority of the diatoms present 

 at the first glance a very striking appearance, each being surrounded 

 by a broad, bright, and sharply defined border, within which the 

 smaller granules of Indian ink do not penetrate. In a surface view 

 this halo usually follows the contour of the frustule at a distance 

 equal to about half the width of the valve ; but when the lateral 

 aspect of the diatom is turned towards the observer, this clear border 

 is seen to be interrupted in a symmetrical manner at both ends of 

 the cell, and also in the vicinity of both central nodes (Fig. 8) ; here 

 the granules of Indian ink approach close to the cell-wall. 



The appearance just described is interpreted by Lauterborn as 

 pointing to the existence of an enveloping layer of hyaline jelly, so 

 remarkably transparent, and possessing a refractive index correspond- 

 ing so closely with that of the surrounding water, as to be com- 

 pletely invisible in clear water, even when examined with the best 

 lenses. 2 Miiller denied the general presence of a gelatinous envelope 

 in the sense advocated by Biitschli and Lauterborn, stating that the 

 clear border only appeared after a long sojourn in the Indian ink 

 emulsion, and that it was absent in the living but completely 

 motionless cell. He, however, subsequently admitted the existence 

 of a gelatinous envelope. In answer to Muller's objections on this 

 point, Lauterborn asserts that the hyaline border becomes visible as 

 soon as the diatoms are brought into the emulsion, and also states 

 that he has often seen Pinnulariae, surrounded by the transparent 

 envelope, remain for hours without the slightest movement. 



The presence of a peculiar and characteristic streaming move- 



1 Lauterborn recommends that the Indian ink should be rubbed up in the water in 

 which the diatoms have been cultivated, and also that specimens from old cultures 

 should not be employed in studying movement. 



2 A gelatinous envelope of similar transparency has been observed in a pelagic form 

 of Cyclotella comta. 



