1010] ANIMAL. PKODUCTION". 867 



factois ill iiiioe — those for agouti liiiir-pattorn, dark eye-color, and blaclv body- 

 pigment — and (2) any selective mortality of particular zygotic types in early 

 postnatal life, a number of mice heterozygous for all three factors were mated 

 to triple reoessives at the Illinois Experiment Station. Most of the data se- 

 cured were from a grotip of 90 Fi wild gray X pink-eyed brown females crossed 

 back to jiink-eyed brown males. These females produced 361 litters (comprising 

 2,259 individuals) in which all the young lived during the two weeks after 

 birth that necessarily elapses before the eye- and body-colors of young mice 

 can be determined, and 170 litters (811 individuals) in which some of the young 

 died during this period. 



No evidence of linkage was found — indeed, in the undepleted series, of the 

 eight classes of young the two that would have been the noncrossover classes 

 were least numerous. There was, however, an indication of selective mortality, 

 for noticeably fewer nonagoutis than agoutis were found among the depleted 

 litters, while the two kinds were substantially equal in number in the Tin- 

 depleted litters. 



riuctuations of sampling' in a Mendelian population, .T. A. Detlefskn 

 (Genetics, 3 (1918), No. 6, pp. 599-607).— The data furnished by the 531 litters 

 from the Fi females used in the experiment noted above were examined to see 

 how closely the ratios in the individual litters approach the expected ratios. 

 The statistical methods adopted are mainly those suggested by Harris (B. S. R., 

 29, p. 67). It is concluded that the observed deviations from expectation can 

 properly be attributed to the chance fluctuations of random sampling. 



A demonstration of the origin of two pairs of female identical twins from 

 two ova of high storag'e metabolism, O. Riddle {Jour. Expt. Zool., 26 (1918), 

 No. 2, pp. 227-25ff ) . — Two cases among ring doves are reported where an egg har- 

 bored two embryos which on dissection proved to be females. The eggs, which 

 were laid by different birds, were very large but are held not to have been double 

 yolked because (1) in one case the two-embryo condition was discovered early 

 in incubation by candling and the relative position of the embryos could not 

 be altered by rotation and shaking, and (2) in both cases the umbilici of each 

 pair of fully formed embryos " had a practically common point of union on 

 the [single, only partly absorbed] yolk-sac." The twins are termed identical 

 because each pair is thought to have been derived from a single egg cell. The 

 large size ("high storage metabolism") of the yolks is suggested as a factor 

 in the separation of the blastomeres to form two embryos. 



Genetic variability, twin hybrids and constant hybrids, in a case of bal- 

 anced lethal factors, H. J. Muller (Genetics, 3 (1918), No. 5, pp. ^22-499, fig. 

 1). — On the basis of extended breeding investigations which he reports with 

 beaded-wing drosophilas, the author recognizes a condition where heterozygosis 

 is enforced by two opposed lethal factors in the same linkage group which are 

 inhibited from crossing over by a third factor, and designates this condition 

 as one of balanced lethal factors. He uses this as an explanation of the 

 existence both of " pure " beaded-wing stock and of stock in which the beaded- 

 wing factor behaves like the factor for yellow in mice. 



The author also points out that the factorial analysis of these flies has 

 proceeded far enough so that various complicated and apparently non-Mendelian 

 results, such as twin hybrids, constant hybrids, and the disappearance of domi- 

 nant factors on crossing, can be made to order. If these cases occurred spon- 

 taneously the Mendelian mechanism at work might riot be apparent, and the 

 phenomena would be made much of by advocates of the theory of factor 

 variability. 



Fluctuations in a recessive Mendelian character and selection, E. Roberts 

 (Jour. Expt. Zool, 27 (1918), No. 2, pp. 157-192, fiys. 25).— Breeding experi- 



