670 ON CONCRETIONS, SPICULES, [ch. 



are all but unknown. It is amorphous in our Ijones. It has the 

 form of calcite in an oyster, a starfish, a Gorgonia, a Globigerina; 

 but of aragonite in most molluscs and in all ordinary corals. It is 

 of calcite in a bird's egg, of aragonite in a tortoise's; of the one 

 in Argonauta, of the other in Nautilus; of the one in an Ammonite, 

 and the other in its Aptychus-\id ; of the one in Ostrea, the other in 

 Unio; of the one in the outer and the other in the inner layers of 

 a limpet or a mussel-shell. Physical chemistry has little to say 

 of the formation of these two, of the parts played by temperature, 

 by the presence of sulphate of hme, or of magnesia or of various 

 impurities; it leaves us in the dark as to what brings the one form 

 or the other into being in the organism*. 



Organic fibres, animal and vegetable, proteid and non-pro teid, 

 hair and wool, silk, cotton and the rest, may be mentioned here 

 in passing: because, as formed material, they have a certain analogy 

 to the spicular formations with which we are concerned. A hair 

 or a wool-fibre may shew upon its surface the scaly or scurfy 

 remnants of the living cells among which its substance was laid 

 down; but the wool itself is by no means. living, but is so nearly 

 crystalline as to shew, in an X-ray photograph, the Laue interference- 

 figures well known to physicists. Moreover, the same identical 

 figure is obtained from such diverse sources as human hair, merino- 

 wool and porcupine's quill. But if we stretch the thread, whether 

 of hair or wool, the first Laue diagram changes to another; one 

 crystalline arrangement has shifted over into a new form of molecular 

 equihbrium. We are deahng with a crystalline, or crystal-hke, 

 form of keratin, the substance of which hoof and horn, nail, scale 

 and feather are made; and this remarkable substance turns out to 

 be a comparatively simple substance after all, with no very high 

 or protein-like molecule |. 



From the comparatively small groun of inorganic formations 

 which, arising within living organisms, owe their form to precipita- 

 tion or to crystallisation, that is to say to chemical or other molecular 



* Cf. Marcel Prenant, Les formes mineralogiques du calcaire chez les etres 

 vivants, Biol. Reviews, ii, pp. 365-393, 1927. 



•f" The study of wool and other fibres has much technical importance, ancj has 

 gone far during the last few years; cf. W. T. Astbury, in Phil. Trans. (A), ccxxx, 

 pp. 75-100, 1931, and other papers. 



