5So 



NATURE 



[July 7, 192 1 



Sex-change in the Native Oyster (O. edulis). 



It is well known that sex-change in the native 

 oyster (O. edulis) occurs at some period of its life. 

 This mollusc apparently always begins life as a male, 

 and may change into a female at the age of one or 

 ,two years. Very little is, however, known about 

 ^the change of sex afterwards. In following up the 

 i';indications given from gf general study of breeding 

 (see J. H. Ortoh, "Sea-temperature, Breeding, and 

 Distribution in Marine Animals," Journal of the 

 Marine Biological Association, vol. xii., July, 1920, pp. 

 339-66), it seemed certain that an oyster ought to con- 

 tinue breeding in the same season even after becoming 

 white-sick, i.e. after extrusion of ova into the mantle 

 cavity. Thus if a breeding oyster were marked and 

 examined afterwards, it should be possible to find 

 out something about a possible annual change of sex. 

 Accordingly on July 30, 1920, two white-sick oysters 

 were isolated in a tank at Plymouth, and one of them 

 was cut open and examined on August 26, 1920. At the 

 latter date the one examined ' was found to have its 

 gonad full of wholly ripe sperm-morulae, which dis- 

 integrated into 'separate active and apparently ripe 

 sperm as soon as they were placed in sea-water. 

 Thus a female-functioning oyster had changed into a 

 male-functioning oyster within less than a month. 

 An indication of this change had already been given 

 on July 29, when the gonad of a white-sick oyster — 

 examined at the moment when it contained embryos 

 in the mantle cavity — showed developing sperm- 

 morulae and some actively tailed sperm-morulae. 



The occurrence of developing sperm-morulae in 

 microscopic sections of "white-sick " or "black-sick " 

 oysters has, indeed, been already observed by P. P. C. 

 Hoek in a practically unknown and very valuable 

 piece of work on the oyster (" Rapport over de 

 Oorzaken van den achteruitgang in hoedanigheid 

 van de Zeeuwsche oester," p. 175. Uitgegenen 

 Door Het Ministerie van Waterstaat, Handel en 

 Nijverheid, 's Gravenhage). 



This year the observations on " white-sick " oysters 

 have been repeated, and all the oysters examined have 

 shown either some sperm-morulae with active tails 

 which disintegrate into separate sperm in sea-water, 

 or developing sperm-morulae. It is seen, therefore, 

 that even at the time an oyster is carrying its own 

 embryos it is changing into a male-functioning form, 

 which will apparently function as a male within a 

 \rery short time. 



An endeavour is being made this summer to carry 

 out on a larger scale the isolation in the sea of 

 oysters of known sex at a particular moment with 

 the view of determining the sex at a later date. It 

 is hoped in this way to investigate also the possible 

 change of an oyster which is male-functioning at the 

 beginning of the breeding season into a female- 

 functioning form at a later period in the same season. 



J. H. Orton. 



Marine Biological Laboratorv, Plymouth, 

 June 18. 



A New Acoustical Phenomenon. 



I HAVE read Dr. Erskine Murray's letter in Nature 

 of June 16, p. 490, with very great interest, but I 

 think there are two difficulties in the explanation that 

 he has there advanced : — 



(i) It is hard to see how or why an aeroplane 

 should emit a series of pairs of double sound im- 

 pulses ; and (2) even if they were emitted, the ear 

 would find it dIfTicult to observe any change, in pitch 

 as the distance from the ground was varied ; for it is 



* The other ipecimen kept for examination this year died at the end 

 of May. 



NO. 2697, VOL. 107] 



found by experiment that pairs of sound impulses 

 cause a sensation of pitch which is sufficient for the 

 identification of a note as being high or low, but is 

 too indefinite for the appreciation of small differences 

 of wave-length. 



I should like to suggest instead that the pheno- 

 mena observed bv Dr. Erskine Murray are due to the 

 presence of a series of stationary sound-waves of 

 various wave-lengths lying parallel to the ground, 

 analogous to the stationary waves of light employed 

 in Lippmann's colour photography. These stationary 

 sound-waves would be produced by reflection at the 

 surface of the ground, the nodes occurring at a dis- 

 tance from the ground inversely proportional to the 

 pitch. 



This suggestion fits in with the observed facts 

 (1) that the note heard varies inversely as the height 

 of the observer's ear from the ground; (2) that the 

 effects are best observed when the aeroplane is nearly 

 overhead; (3) that the note heard at a given height 

 varies with the angle of elevation of the aeroplane ; 

 and (4) that the surface of the ground must be 

 smooth. 



As to the source of these series of notes of different 

 wave-length, it would seem that the turbulent air 

 behind wings, framework, and propeller must be 

 responsible, and the fact that wind passing through 

 a tree can create similar phenomena would seem to 

 confirm this view. With regard to the physiological 

 aspect, it has long been known that double sound 

 impulses do give a crude sensation of pitch, and both 

 theories of hearing have offered suggestions to account 

 for it. H. Hartridge. 



King's College, Cambridge. 



During the war and since I have often noticed 

 how the apparent pitch of aeroplane noise changes 

 suddenly as an aeroplane travels over the street in 

 which one is standing. I had put this down to 

 reflection, but not on the lines followed by Dr. 

 Erskine Murray in his letter in Nature of June 16, 

 p. 490. As the problem is of practical importance to 

 such bodies as the War Office and Admiralty, in, for 

 example, recognising aircraft at night or in fogs, it 

 seems worthy of discussion. 



If the sound from an aeroplane were a pure tone 

 no amount of reflection could give the sensation of 

 the octave, for two harmonics of equal period combine 

 into an harmonic of the same period. If the sound is 

 impure and has overtones, combination of direct and 

 reflected waves could have the effect only of altering 

 the quality by suppressing some components and re- 

 inforcing others. I suggest that Dr. Murray heard 

 the upper tones because of interference between the 

 direct and reflected waves of the lower. That the 

 noise from an aeroplane, though often of musical 

 quality, is not a pure tone is clear. Exhaust noise, 

 in spite of the approximately harmonic motion of the 

 pistons and valves, is not a pure tone. Complications 

 arise from the explosive emission of the gases. 

 Moreover, in addition to the dominant exhaust noise, 

 there are secondary noises from propeller, fuselage, 

 etc. H. S. RowELL, 



Director of Research, The Research Asso- 

 ciation of British Motor Manufacturers. 



15 Bolton Road, Chiswick, W.4, June 20. 



The acoustical phenomenon described by Dr. 

 Erskine Murray in Nature of June 16, p. 490, is 

 fully discussed by F. A. Schulze in a paper which 

 appeared in the Annalen der Physik in 19 16 (vol. xllx., 

 p. 683). References to earlier work on the subject are' 



