August 31, 1917] 



SCIENCE 



213 



several years. Publication of otlier methods 

 for obtaining a large and more or less con- 

 tinuous supply of these animals has not been 

 infrequent and many are familiar with use of 

 Elodea (Philotria, Michx.-Britton), Cera^o- 

 phyllum and other aquatic plants. 



The ditch-moss is not readily found in many 

 localities. My personal experience with sev- 

 eral aquatic plants yielded indifferent results 

 and failed to give sufficient numbers until, by 

 chance one season, I tried the marsh plant, 

 Elodes campanulata {Triadenum virginicum 

 (L.) Eaf., see Britton and Brown) and was 

 rewarded with large numbers of amoebfe. Al- 

 though absence from town in some seasons 

 occasioned a too long interval between the 

 times of collection and the use of the material, 

 or made it impossible to provide the proper 

 sequence of cultures, I have seldom been dis- 

 appointed in finding the animals, though they 

 may not have come just when wanted. 



The usual custom was followed in making 

 up the cultures. Crystallizing dishes or bat- 

 tery jars — the shallower dishes gave the better 

 results — were crowded not too densely with 

 the stems of the plants. The stems were 

 usually cut two or three times. Tap water 

 and water from the pond or marsh where the 

 plants were collected were used, separately, 

 but no difference in results was noted. The 

 dishes were covered with plates of window 

 glass, placed in a room of moderate temper- 

 ature and there allowed to remain in diffuse 

 light for a period of three weeks or more. 

 When pains were taken to collect the plants 

 at intervals and provide a sequence of cul- 

 ures the results were most gratifying. 



I have used the plant from four different 

 localities, collecting from the water and from 

 banks where the plants could only have been 

 submerged at high water and mixing, with 

 success in all cases. Since the locality seems 

 not to be a controlling factor, and since the 

 cultures of tap as well as iKjnd water yield 

 the animals, I assume that the Elodes is favor- 

 able for the original lodgnient of amoebffi and 

 their later multiplication. 



C. E. Gordon 



Amhebst, Mjvss. 



CROSSING-OVER IN THE SEX CHROMOSOME 

 OF THE MALE FOWL 



Setoral years ago an experiment was begun 

 with the object of studying the inheritance 

 of several sex-linked characters associated in 

 the same individual, but the experiment had 

 to be laid aside until last year. The second 

 generation chicks are now at hand and prove 

 beyond doubt that crossing-over takes place be- 

 tween the sex chromosomes of the male fowl. 



In this preliminary report attention will be 

 confijied to the factors themselves, without 

 regard to the somatic appearances of the in- 

 dividuals. Three dominant sex-linked char- 

 acters, viz., B, I, and S were employed. B 

 and I were introduced on one side; S, on the 

 other. Hence the F^ males were all BI, S,; 

 B and I being in paternal (or maternal) sex 

 chromosome, S in the maternal (or paternal). 

 These males have been tested by mating them 

 back to females of the composition b Is, b is. 



If there were no crossing-over, offspring of 

 this back cross showing the combination of 

 somatic characters found in the Fj male, 

 would not occur. Actually, however, they do 

 occur, thus demonstrating that crossing-over 

 has occurred, a chromosome having the com- 

 position BIS, having been formed. Other 

 cross-over classes have appeared, but the one 

 cited is the one at the present age of the 

 chicks, most easily recognized. 



No crossing in the female is to be expected 

 on theoretical gi-ounds. l^one was observed 

 in the original cross. Partly because of prac- 

 tical reasons and partly because no new com- 

 binations were available in F„ it seemed wise 

 to defer a test of this ix>int until next season, 

 when the new combination BIS should be 

 available in the mature female. 



H. D. GOODALE 



Massachusetts Agricultural 

 Experiment Station 



the equal parallax curve for frontal 

 and lateral vision 



In the article by Mr. C. C. Trowbridge on 

 " The importance of lateral vision in its rela- 

 tion to orientation"^ is given an equal paral- 

 lax curve showing the distances that a man 



1 Science, N. S., Vol. XLIV., No. 1135, pp. 470- 

 474, September 29, 1916. 



