45 



A few years ago Mr. Fischer at Kiel again sent me some material of Pit. 

 indicum, which had thus during many years been cultivated in his laboratory. There 

 was a considerable difference, compared to my stock, but the normal form and the two 

 variants obscurum and parvum, I could still obtain from it as constant forms by means 

 of colony-selection. 



At the examination of numerous samples of seawater in all the seasons, taken near 

 Scheveningen, Bergen op Zoom and den Helder, partly far from the coast 1 ), I have 

 never found Ph. indicum itself, but, even three times, forms which, with a broad 

 conception of the species, might be considered as varieties of it, and else as very 

 closely allied species. I call them Ph. splendidum and Ph. splendor marts. Short after 

 the isolation already they produced variants, one of which is quite dark and 

 multiplies in such a number that in cultures which are negligently re-inoculated 

 the normal form, and with it the photogenic power, wholly disappear. 



Thus, a culture at 22 C. of splendor maris going out from a single phos- 

 phorescent colony, after being in 12 days six times re-inoculated on fish-broth-gelatin, 

 produced 1800 dark variants on 22 colonies of the normal form. The culture re- 

 inoculated six times in the same space of time on fish-agar did not yet contain any 

 variants, whilst at the i2th re-inoculation on agar their number was also very great. 

 The first not further re-inoculated cultures on gelatin, which accordingly had only 

 had little opportunity to grow, after 12 days did not contain variants, in accordance 

 with the rule that at cessation of growth no variability is manifested. 



The />ort'j<ra-variant also is in Ph. splendidum and Ph. splendor maris as 

 distinctly recognisable as in Ph. indicum itself, and here too, frequently produces the 

 primitive forms as atavists. 



Basing on these experiences I think it probable, that the cause which calls forth 

 the variants is not exclusively acitve in our artificial cultures, but can also be active 

 in the sea itself, so that in this case there is a chance that dark forms, isolated from 

 the sea will at first be taken for particular species, but after more minute observation, 

 will prove to be variants of known phosphorescent bacteria. 



By observing certain general conditions the production of dark variants can, as 

 said, be greatly slackened, but not wholly prevented. Among these are: strong nutrition 

 and vigorous growth a little below the optimum temperature, free access of oxygen, 

 such as can be attained in cultures on agar-agar, and total exclusion of the influence 

 exerted by the secretion-products of the bacteria themselves, which is attainable by 

 re-inoculating the young cultures very often on a new medium. 



7. Conclusion. 



I will begin with pointing to the fact that hereditary variability is a function of 

 growth, in particular of slackened growth, but that at cessation of growth no change 

 takes place. And furthermore that variability attacks only one independent charac- 



') Many of these samples I owe to the kindmess of Dr. Hoek. Various species of 

 luminous bacteria have been found in them to the amount of o.i to 5. even 7 pCt., of 

 all present bacteria. Especially Ph. luminosum, and a species difficult to distinguish from 

 it, but still quite different, Ph. hollandiae, occur very often. Ph. degenerans also is frequent. 



