November 24, 1905.] 



SCIENCE. 



675 



help us if we assume a greater vigor of the 

 vs^hite germ cells, so that unions do not take 

 place in hap-hazard fashion, but tvpo germ 

 cells bearing black are less apt to get together 

 than tvpo bearing white, pure black zygotes 

 being produced in less than one fourth of the 

 cases. 



We may conclude, then, that while the 

 third criterion of recessiveness is imperfectly 

 met this does not militate against the reces- 

 siveness of black in the Mendelian sense, but 

 indicates the presence of a second, disturbing, 

 factor. 



The fourth criterion is the least critical be- 

 cause of the impossibility of judging whether 

 a dominant is homogametous or not, except by 

 its performance; if the hybrids are not domi- 

 nants we conclude that the parent is not a 

 pure dominant. The existence, however, of 

 white individuals which always throw whites 

 when mated with blacks is significant in rela- 

 tion to this criterion. Three white parents, 

 descended, so far as known, from white an- 

 cestors, produced, when crossed with black 

 sheep, 13 offspring, all white. 



A special case deserves particular mention. No. 

 907 is a white male both of whose parents, 606 

 and 810, are also white, but both of whose grand- 

 fathers are black. Consequently, 606 and 810 

 are heterogametous but, until tested, we have no 

 means of knowing whether their son, 907, is 

 heterogametous or has only white germ cells. 

 When No. 907 was crossed with heterogametous, 

 white females all offspring were white. This 

 would indicate that No. 907 is homogametous. 

 When No. 907 was, however, crossed with pure 

 recessives (blacks) one out of five offspring was 

 black, and when crossed with ' extracted ' reces- 

 sives (having one heterogametous white parent) 

 it produced two black offspring out of 18. In 

 relation to these three offspring out of 23, as- 

 suming the record to be correct, No. 907 acts as if 

 heterogametous. The occasional appearance of 

 black offspring from a homogametous and a 

 heterogametous parent may be explained as an 

 occasional prepotency of black over the dominant 

 white, — a phenomenon described by Castle^ ( 1905, 

 p. 58 et folg.). 



- Castle, W. E., 1905, ' Heredity of Coat Charac- 

 ters in Gruinea-Pigs and Rabbits.' Papers of Sta- 

 tion for Experimental Evolution at Cold Spring 

 Harbor, New York, No. 2. 



It may be concluded that the fourth cri- 

 terion also speaks for the recessiveness of 

 black, the only exceptional case being ex- 

 plained on special grounds. 



The conclusion of the whole matter is that 

 black wool color in sheep behaves like a 

 Mendelian recessive characteristic. 



C. B. Davenport. 

 Department of Experimental Biology, 

 Carnegie Institution, 



Station foe Experimental Evolution, 

 Cold Spring Harbor, N. Y. 



phototropism in the larval and early ado- 

 lescent STAGES OF HOMAEUS AMERIOANUS. 



In view of the interest which, by the recent 

 excellent work of Keeble and Gamble on the 

 color physiology of higher Crustacea, has been 

 renewed in the study of the effects of light 

 upon many forms of littoral Crustacea, the 

 following results obtained during the past 

 summer, in a series of experiments upon the 

 effect of light on the larval and early adoles- 

 cent stages of the American lobster, may be 

 appreciated by some who are engaged in in- 

 vestigation of a similar nature upon other 

 forms. The records of the following experi- 

 ments cover but a small part of the field of 

 inquiry into the effect of light upon these 

 forms, in so much as they do not take up the 

 subject of the influence of light upon chro- 

 matophore activity or pigment movement, but 

 merely attempt to describe the reactions of the 

 first five stages of Homarus americanus to 

 light, upon backgrounds of black and white. 



The apparatus used for the experiments 

 consisted of an oblong wooden box whose in- 

 side dimensions were 6x18x4 inches (deep). 

 The box was black inside and fitted with a 

 light-tight cover, through one end of which 

 protruded, to a length of 6 inches, a card- 

 board tube, 1^ inches in diameter, the func- 

 tion of this tube being to admit none but 

 nearly parallel rays of light into one end of 

 the box, thus distinctly localizing the light 

 area. In cases wherein a white background 

 was required, the black interior of the box was 

 merely covered with white paper, as was the 

 inside of the light-tight cover.^ 



^ The design of the box is based upon that of 

 Keeble and Gamble. 



