552 



NA rURE 



[April 5. 1900 



distinction between cytoplasmic and nuclear maturation ; he 

 reared a merogonic sea-urchin larva whose cells had the normal 

 number (l8) of chromosomes, although the spermatozoon- 

 nucleus (the only one in this case) imported (it is presumed) but 

 half that number. The last fact leads him to conclude that the 

 number of chromosomes is a specific property of the cell. 



Although Delage's experiments stand at present alone as 

 regards the method pursued, there have been of late a number 

 of experimental studies on fertilisation, all of which present 

 .points of great interest. From among these we select those of 

 Prof. Jacques Loeb,' as it seems of particular importance that 

 Ihis results should be collated with those of Delage. 



Loeb finds that the mixture of about 50 per cent, of y n 

 MgClj with about 50 per cent, of sea-water is able to bring 

 al)out (in the eggs of the sea-urchin Arbacia) the same result as 

 the entrance of a spermatozoon. After being subjected to this 

 mixture for about two hours, the eggs were returned to normal 

 sea- water, wherein many developed, forming blastulse, gastrulse 

 and plutei. Fewer eggs developed than in natural conditions, 

 and the development was slow, but otherwise the results were 

 normal. The author believes that the only reason why the eggs 

 of this sea-urchin and of other marine animals do not usually 

 develop parthenogenetically is the presence or absence of ions of 

 sodium, calcium, potassium and magnesium. The two former 

 require to be reduced, the two latter to be increased. 

 ■ " The unfertilised egg of the sea-urchin contains all the 

 essential elements for the production of a perfect pluteus." 

 All the spermatozoon needs to carry into the egg for the process 

 of fertilisation are ions to supplement the lack of favourable 

 ions, or to counteract the effects of the other class of ions in the 

 sea-water, or both. "The spermatozoon may, however, carry 

 in addition a number of enzymes or other material. But the 

 ions and not the nucleins in the spermatozoon are essential to 

 the process of fertilisation." 



It is interesting to observe that while Delage's experiments go 

 to show that the nucleus of the sea-urchin ovum is not essential 

 to development, Loeb's experiments go to show that the sperm- 

 atozoon may (with intact ova) be dispensed with. What is now 

 needed is a combination of the two modes of experiment. 



J. A. T. 



CHANGES OF COLOUR OF PRAWNS. 

 TT has long been known that the very numerous varieties of 

 ■*■ the prawn Hippolyte ( Virhius) varians reflect, each after its 

 kind, the colour of the weed or zoophyte to which they cling, 

 -and on which they find both food and shelter. A few natur- 

 alists, after noting this striking case of " protective resemblance," 

 have detached some of the more brilliantly coloured specimens 

 for the purpose of making a detailed subsequent examination. 

 When they came to do this they found that the vivid brown and 

 other tints had in the interval largely faded, or were replaced 

 by others. This discovery has no doubt been made in- 

 dependently time after time, and has given point and emphasis 

 to the essentially variable character of this prawn. Not only 

 do individuals differ from each other, but any one of them is 

 •capable of altering its characteristic tint. 



Thus, at the time when Keeble and Gamble began their 

 observations,^ Hippolyte varians was known to change colour, 

 but while one author stated that a sympathetic colour-change 

 was rapidly effected, as well in the dark as in the light, when 

 weed of a new tint was introduced ; another affirmed that even 

 in the light the change was slow and did not always agree with 

 the colour of the new weed. Yet a third author stated that 

 darkness by itself has a distinct reddening effect. The only 

 definite conclusion to be drawn from these curiously conflicting 

 statements was that Hippolyte offered a fine field for research, 

 and that though a few strollers had here and there plucked an 

 ear or two of corn, there was a fine harvest still to be 

 gathered. 



After two years' work on the coasts of Lancashire and of 

 Normandy, Keeble and Gamble have come to the conclusion 

 that three kinds of colour-change may be distinguished in 

 Hippolyte. 



1 On the nature of the process of fertilisation and the artificial production 

 of normal larvae (plutei) from the- unfertilised eggs of the sea-urchin. 

 (Anter. Joiirn. Physiol, iii. (iSgg), pp. 135-138.) 



2 " The Colour-Physiology oi Hippolyte varians." By F. W. Keeble, 

 Caius College, Cambridge, and F. W. Gamble, Owens College, Manchester. 

 Read before the Royal Society on November 23, 1899. 



NO. 1588, VOL. 61' 



I. First, a periodic and rhythmic cycle of change composed o. a 

 diurnal and a nocturnal phase of colour. Towards evening a 

 decided red tinge — a sunset glow — makes its appearance, and 

 this ushers in the nocturnal change. A green tinge ensues, 

 which spreads fore and aft from the middle of the body. 

 Presently this green colour gives place to an azure-blue colour, 

 which is the characteristic nocturnal tint, and is accompanied by 

 a greatly heightened transparency in the tissues. Under 

 natural conditions this colour-phase persists until daybreak. At 

 the first touch of dawn it disappears, and that of the previous 

 day is gradually reassumed. 



More striking even than the distinctive colours is the periodi- 

 city of the nocturnal and diurnal phases. Thus, in constant 

 darkness a nocturne (that is a prawn in the nocturnal colour- 

 phase) recovers to its diurnal colour. In constant light, a 

 diurnal form passes over to the nightly phase. Though light 

 often induces, and induces with marvellous rapidity, a recovery 

 from the nocturnal colour to that of the previous day, yet it is 

 often powerless to overcome the habit of the animal. The 

 periodicity is only worn down in the course of two or three 

 days. 



It follows that since the colour of Hippolyte is a function of 

 the time of day, that time must be taken into account in an 

 investigation on the colours of Crustacea. 



II. The second kind of colour-change is the susceptibility 

 of Hippolyte to changes of light-intensity. Although the periodic 

 habit of the prawn is the hitherto unknown and yet dominant 

 factor, yet its force is greatest at the times of the assumption 

 of the nocturnal phase, and the resumption of the diurnal tint. 

 At other times external conditions may modify the colour of the 

 animal to a large extent, and the chief agent in the production 

 of these modified colours is the varying amount of light reflected 

 from or scattered by, surrounding objects. 



An almost black prawn changed in a few minutes, after being 

 placed in a white porcelain, to a transparent and colourless con- 

 dition. Further, a ready and almost infallible means of pro- 

 ducing green prawns is to place them just after their capture in 

 a white jar, and cover the mouth of the vessel with muslin. 

 Under these circumstances the change^— from brown to green, 

 for example — takes place in from thirty seconds to one minute. 



Speaking generally, exposure to a low light-intensity during 

 the day favours an expansion of the red pigment, and so pro- 

 duces brown or even reddish effects. Hence, probably the red 

 colour of these prawns at sunset ; while an increase in the 

 amount of light, especially if scattered from a white smooth 

 surface, produces a green effect by expanding the blue and 

 yellow pigment and causing the red to contract. 



III. The third change differs chiefly from the second in its 

 rate of progress. It is the very slow sympathetic colour- 

 change which ensues when adult prawns, taken from a food- 

 plant of one colour, are placed with the weed of a new colour. 

 Thus, if green Hippolyte be placed with brown weed, and the 

 light-intensity maintained unaltered, as far as possible in com- 

 parison with the light-conditions of its former habitat, the 

 prawns will retain their green colour even for a week or itiore, 

 but in the end give way and become brown. Their subsequent 

 recovery when placed with green weed is more rapid. Keeble 

 and Gamble have repeated such experiments time after time in 

 the open, and under as natural conditions as possible, and found 

 that the prawns were either quite refractory or responded in 

 this slow manner. Yet these same specimens, as each evening 

 drew on, underwent the colour-changes culminating in the noc- 

 turnal hue with the greatest readiness, and recovered as quickly 

 the next morning to the tint of the previous day. 



The great difficulty in ascertaining whether Hippoiyte re- 

 sponds to change in the colour of its surroundings by a sym- 

 pathetic change of its own bodily tints is now clear. It lies in 

 their marvellous sensitiveness to changes of light-intensity, as 

 apart from colour, and is increased by the dominant and periodic 

 colour-changes which subvene night and inorning. If it were 

 possible to eliminate these two factors, then we might be able to 

 detect the response of Hippolyte to colour or change of colour 

 per se ; in fact, Keeble and Gamble have made an attempt. By 

 the use of colour-screens, based on the instruments used by 

 Landolt and other workers, the prawns are subjected to red, 

 green and blue light, and also a width of a spectrum from the 

 red to the green. The results of these experiments are curious. 

 They show that even when the light transmitted by the screen, 

 and falling on the prawns, is higti (the incandescent lamp and 

 a mirror being used to effect this), yet that with red, green and 



