Z30L0GY AND BOTANY, MICROSCOPY, ETC. 53 



in the hybrid, and frequently in its germ-cells also. This balanced 

 condition, once obtained, is stable under close breeding, for the germ- 

 cells are not D or R, but |DR, and this breeds true to itself ; but this 

 is very easily disturbed by cross-breeding, e.g. if the gamete unites 

 with a pure R or a pure D, the result is D (R) ; in some cases it may 

 be J D (R) • R, and this latter hypothesis accounts for the reappearance 

 of, say, spotted mice after their disappearance for a generation in con- 

 sequence of crossing. 



(2) Stable hybrid forms may occur, and this occurrence may be 

 explained either by the gametes bearing the balanced relationship | D R, 

 or by one of the gametes which unite, bearing the character D and the 

 other the character R. 



(3) Coupling (or complete correlation) may exist between two or 

 more characters, so that they form a compound unit not separable in 

 heredity, e.g. the blue flower and purple-coloured stems of Datura, 

 and in animals, white hair and pink eyes. 



(4) Disintegration of characters apparently simple may take place 

 in consequence of cross-breeding. Thus the grey coat-colour of the 

 house-mouse is always transmitted as a dominant unit in primary crosses 

 with its white variety, but in the second generation a number of black 

 appear. These black mice belong to the category of dominant in- 

 dividuals, but they have only the black constituent of the grey coat ; the 

 remaining constituent, a rufous tint, has become separated, and it may 

 have become latent (recessive). 



(5) Departures from the ratios of dominants to recessives may be 

 explained in some cases as due to inferior vigour, and so greater 

 mortality, on the part of dominants or recessives respectively. 



(6) Cases of apparent reversal of dominance may arise from " false 

 hybridisation " or induced parthenogenesis, where the one gamete has 

 served merely to stimulate the other to development without uniting with 

 it. It is possible, however, that one of a pair of characters may be 

 sometimes dominant, sometimes recessive. Sex may be a case of this 

 sort. 



Mendel's principles strengthen the view that species arise by dis- 

 continuous variation. They explain also why new types are especially 

 variable, how one variation causes others, and why certain variations 

 are so persistent in their occurrence. 



Regeneration of Hind Limbs and Tail in Amphibia.* — P. Wintre- 

 bert finds that in the larvse of Anura -regeneration of the tail is rapid 

 and regular, and independent of the nervous system. In Siredon, after 

 spinal extirpation, it proceeds regularly. His conclusion is that regenera- 

 tion and ontogeny proceed in the same fashion ; the various parts are 

 formed in the same order in both cases, and apparently independently of 

 nervous control in the ordinary sense. 



Interstitial Cells of the Mammalian Testicle.f — P. Bouin and 

 P. Ancel have investigated the morphological and functional relations of 

 the testis and its interstitial cells, and make out an independent function 

 for the latter. They find that interstitial cells exist in the Mammals 



* Comptes Kendus, cxxxvii. (1003) pp. 761-3. 

 t Arch. Zool. Exp., 1903, pp. 437-523 (3 pis.). 



