493 



thequestion of the number of embryos, which may arise 

 in one chorion. 



A full discussion of identical twins, etc. may be reserved for 

 another occasion, here it need only be stated, that their occurrence is 

 probably more frequent, than has hitherto been supposed. By competent 

 authorities it has been estimated, that in man identical twins form 

 25 per cent of all twins. Their frequency alone is against the idea 

 of their occurrence being due to, say, a chance division (partition) of 

 the developing egg. 



Extremely improbable, if not impossible, is the origin of one of 

 them from a fertilised polar body. As little can one hold this, as 

 accept the idea of "chance" in the development. In some other 

 mammals identical twins would appear to be very common. Thus, in 

 the sheep, where the total number of young is usually two or three, 

 the writer has more than once come across them in utero. There 

 exists, however, a case in the literature, that of Praopus hybridus, 

 where according to v. Jhering x ), 8 and more young are formed within 

 each chorion, i. e., as products of one egg. The observation tallies 

 with and is allied to that already recorded by v. Koelliker 2 ) in 

 1879, of 4 foetuses within one chorion in a related species, Dasypus 

 (Praopus) novemcinctus. As v. Jhering remarks, it shows how little 

 reason there is for the common belief, that it is an invariable rule, 

 or even law, for only one embryo to arise from a single egg. The 

 occurrence and comparative frequency of identical twins, triplets, etc. 

 in man, taken along with the above observation and other con- 

 siderations, point to a former multiplicity of embryos formed as the 

 progeny of one egg, even in the ancestry of man, as also urged by 

 v. Jhering. 



I would now take a further step, and insist, that the tumours, 

 including cancer 3 ), date back to this condition as their ultimate source. 



1) PL v. Jhering-, Ueber 'Generationswechsel' bei Säugetieren. 

 Biol. Oentralbl., Bd. 6, 1886, p. 532—539 (vide also: Berl. Sitzungsb. 

 and Arch. f. Physiol., 1886). 



2) A. v. Koelliker, Entwicklungsgeschichte des Menschen, 2. Aufl., 

 1879, p. 362. 



3) I have already elsewhere (Lancet, 1902, p. 1760) shown, that carci- 

 noma, as due to the unrestricted pathological growth of the represen- 

 tative of a chorion or trophoblast, is a disease possibly peculiar to mammals, 

 and a sequel to uterine gestation. Sarcoma, on the other hand, as re- 

 presenting a much reduced embryo (embryonic tissue of a reduced 

 embryoma) may be found lower down in the scale, and, in fact, Prof. 

 Adami has informed me of a case of sarcoma in a codfish, Dr. J. A. Murray 



