210 



SCIENCE 



[N. S. Vol. XXV. No. 632 



Ceylon pearls might thus be grown in other 

 parts of the world. Kelaart says that the 

 nucleus of an American pearl, drawn by 

 Mobius, is of nearly the same form as that 

 found in the pearl oyster of Ceylon. 



It is interesting to note in comparison the 

 record of Johnstone that in the gurnard 

 small pearl-like bodies are found adhering 

 to the peritoneal investment of the intes- 

 tine. The concretion is made up of a great 

 number of concentric lamelliE which seem 

 to consist of wavy bundles of connective 

 tissue. These structures are probably de- 

 rived from Tetrarhynchus cysts from which 

 the larvae had escaped and which had then 

 undergone calcareous degeneration. 



The amount of growth may be much 

 greater than thus far indicated and a condi- 

 tion among animals closely analogous to 

 the formation of galls on plants has been 

 observed by Nutting. In certain Alcyo- 

 naria a tunnel formed of excessively en- 

 larged spicules is found along one side of 

 the stem or branch. The abnormal struc- 

 ture is due to an annelid. Greatly en- 

 larged polyps in another genus owe their 

 origin to the presence of Crustacea or some 

 other form of parasite. 



Continued efforts have been made to 

 demonstrate that the extensive pathological 

 growths found in animals are due to the 

 stimuli exerted by minute parasitic forms. 

 It has been shown that in plants such ab- 

 normal growths are due to the encroach- 

 ments of parasites; but the effort to iden- 

 tify animal organisms as the cause of 

 cancerous and other abnormal growths has 

 in the opinion of most investigators failed 

 to establish a case. 



When we seek to ascertain the causes of 

 the morphological changes which result 

 from the influence of the parasite on its 

 host it is difficult in the present state of 

 knowledge to find any very definite ex- 

 planation. Localized growth is ordinarily 

 a factor of differentiation, but here it has 



no reference whatever to that process. 

 Davenport in his admirable treatise on ex- 

 perimental morphology has given a very 

 clear discussion of the factors in the effect 

 of chemical and physical agents upon 

 growth, and has brought together the evi- 

 dence which shows the acceleration of 

 growth by contact, by cutting and by 

 chemical stimulus. Now an examination 

 of the cases in which growth is induced by 

 parasites leads to the conviction that 

 neither contact nor cutting can be a stimu- 

 lating factor, since there are too many 

 cases of parasitism in which no growth is 

 produced, while in the exceptional ease the 

 presence of the parasite stimulates con- 

 siderable gTowth activity. To be sure, it 

 has already been suggested that the forma- 

 tion of a cyst about a resting parasite is a 

 normal activity of the host and occurs very 

 generally if not universally. This may be 

 the result of a contact stimulus; it may 

 also be explained on the same basis as the 

 other cases, viz., as a result of chemical 

 stimulus. Under the discussion of the 

 acceleration of growth by chemical stimu- 

 lus, Davenport says : " It is clear from this 

 table that the addition of even small quan- 

 tities of innutritions and poisonous sub- 

 stances may so affect the hylogenie 

 processes as to cause twice or even far more 

 than twice the normal formation of dry 

 substance in a given time, and that this ex- 

 cessive growth increases with the concen- 

 tration of the poisonous substances up to 

 a certain optimum beyond which growth 

 declines again to below the normal." 



In suggesting that the stimulation of 

 growth by parasites is traceable to chemic- 

 al stimulus, and that the stimulating sub- 

 stance is a poison, I am advancing a hypo- 

 thesis which, even though it is a purely 

 tentative one, may well engage our con- 

 sideration for a moment. Each parasite in 

 the course of its activities produces a cer- 

 tain amount of waste material. It may 



