July 14, 192 1] 



NATURE 



621 



other words, it may grow too fast and eventually kill 

 the host. 



Do the pearl elements ever behave so in the induced 

 variety ? * Should any positive evidence of this be 

 available, it would throw much valuable light upon 

 the ontogeny of cancer. 



The view that warts, and even cancers, are trans- 

 plantable is strongly supported by the artificial induc- 

 tion of pearls. 



Wyatt Wingrave, 

 Consulting Pathologist, Central London 

 Throat and Ear Hospital. 

 Lyme Regis, Dorset, July 8. 



I DIRECTED attention in my 1902 paper (Proc. Zool. 

 Soc., March 4, 1902)10 the resemblance between pearls 

 and " the structures sometimes found in epidermoid 

 tumours and atheroma cysts." .\ pearl might be 

 compared to the concentrically deposited ball of 

 desquamated epithelial cells characteristic, I believe, of 

 the latter, except for the fact that the pearl (like the 

 normal molluscan shell-substance, and unlike the 

 outer laver of the skin, and the nails, horns, hair, etc., 



n efi 



LWHjcuiJiiyanBDniiii^ 



Fig. 1. — The pearl-inducing trematode Gymno- 

 phaVus dapsilis or G. hursicola in the 

 sub-epidermal connective tissue of Mytilus, 

 surrounded by an epidermal sac which 

 becoii es the pearl-sac : o. ep., outer shell- 

 secretins epidermis ; ep. sac., epidermal pearl- 

 sac; par., parenchymatous connective tissue; 

 bl. c, blood-cells; (rem., trematode. The sac 

 is usually about o"4 to 0*5 mm. in diameter. 



in mammals) is not composed of cells, but secreted 

 at the surface of cells. 



I cannot agree that the difference between a blister 

 and a pearl is one of degree and not of kind, as Dr. 

 Wingrave seems to suggest; in spite of the fact thai 

 the nature of the secreting cells, and of the substance 

 they secrete, is identical. The blister is the normal 

 response of the outer shell-secreting epidermis to the 

 mechanical stimulation of any body that comes in con- 

 tact with it. In this sense it resembles a corn on the 

 human foot, or the thickenings of the skin on a navvy's 

 hands. On the other hand, recent evidence goes 

 to show that the sac, or "island," of epidermis in 

 which the pearl is formed arises only in certain quite 

 .'Specific circumstances. In the case of the edible 

 mussel the "circumstance" is probablv the .specific 

 stimulation (quite likely of a chemical nature) of the 

 trematode Gymnophallus dapsilis or G. hursicola. 

 These worms normally become surrounded by such a 

 sac in Mytilus (Fig. i), and when the worm dies, or 

 leaves the sac. a pearl is formed in it. A smaller 

 trematode, which I have not identified, also 

 occurs in the sub-epidermal connective tissue 



1 The step from a " pearly" wart to a " pearly" or nested epithelioma is 

 a very short one. 



of the mussel ; but this species, which is surrounded 

 by a cyst, probably secreted by the worm itself 

 (Fig. 2), and not by an epidermal sac, does not, in 

 my experience, give rise to pearls. Similarly the 

 cestode larva {Tylocephalum ludificans), which was 

 wrongly identified by Herdman, Shipley, and Hornell 

 as concerned with pearl formation in the Ceylon pearl 

 ovster, is surrounded by the oyster with a fibrous con- 



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ttetti. 



^.^. 



Fig. 2. — A smaller (unidentified) trematode in the 

 sub-epidermal connective tissue of Mytilus 

 which is surrounded by a cy^t, probably 

 secreted by the worm itself. The mollusc 

 does not surround the worm with an epidermal 

 sac, and there is no evidence that this species 

 of trematode ever 1 ecomes the centre of a 

 pearl : got*., gonad. Other letters as in Fig. i. 

 The cyst is about o"i5 mm. in diameter. 



nective tissue capsule (Fig. 3), and does not appear to 

 possess the power of provoking the mollusc to produce 

 the epidermal sac in which alone a pearl can be 

 formed. 



In the case of the Mikimoto pearls and of the 

 pearls artificially produced by -Alverdes, the special 

 "circumstance" is the performance of a particular 

 transplantation of tissue. 



One of the facts which have favoured the survival 



Fig. 3. — Solex of the cestode 'fylocephniutn lud ficans in the conUec- 

 tive tissue of the Ceylon pearl oyster. The mollusc surrounds this 

 worm with a capsule (caps.) of fibrous connective tissue instead of 

 with an epidermal sac. In spite of statements to the contiary, no 

 satisfactory evidence has been adduced that this worm is associated 

 with pearl formation. Letier* as in Fig. i. The entire capsule may 

 measure as much as 2 or •; mm. in diameter. 



of the theory that the .same kind of mechanical stimu- 

 lation that produces a blister can produce a pearl-sac 

 and a pearl is the occasional presence in fine pearls of 

 grains of sand and other foreign bodies. I recorded and 

 figured several such instances, from Ceylon pearls, in 

 my 1912 paper (Proc. Zool. Soc., 1912, pi. xlii.. Fig. 38; 

 pi' xliii., Figs. 44, 45; pi. xlv., Figs. 54, 54a; pi. xlvi., 

 Figs. 55, 56). I .suggest the following possible explana- 



NO. 2698, VOL. 107] 



