April 28, 1916] 



SCIENCE 



613 



twin pregnancies of sheep that I have so far 

 had the opportunity to examine, a monochorial 

 condition was found, though the fetuses were 

 dizygotic; but the circulation of each fetus 

 was closed. This appears to be the normal con- 

 dition in sheep; but if the two circulations 

 should anastomose, we should have the condi- 

 tions that produce a sterile free-martin in 

 cattle. The possibility of their occurrence in 

 sheep is therefore given. 



The fertile free-martin in cattle may be due 

 to cases similar to those normal for sheep. 

 Unfortunately when the first two cases of nor- 

 mal cattle free-martins that I have recorded, 

 came under observation I was not yet aware of 

 the significance of the membrane relations, 

 and the circulation was not studied. But I 

 recorded in my notebook in each case that the 

 connecting part of the two halves of the 

 chorion was narrow, and this is significant. In 

 the third case the two chorions were entirely 

 unfused; this case, therefore, constitutes an 

 experimentum cruets. The male was 10.4 cm. 

 long; the female 10.2 cm. The reproductive 

 organs of both were entirely normal. The 

 occurrence of the fertile free-martin is there- 

 fore satisfactorily explained. 



The sterile free-martin enables us to dis- 

 tinguish between the effects of the zygotic 

 sex-determining factor in mammals, and the 

 hormonic sex-differentiating factors. The 

 female is sterilized at the very beginning 

 of sex-differentiation, or before any morpho- 

 logical evidences are apparent, and male 

 hormones circulate in its blood for a long 

 period thereafter. But in spite of this the 

 reproductive system is for the most part of 

 the female type, though greatly reduced. 

 The gonad is the part most affected; so much 

 so that most authors have interpreted it as 

 testis; a gubernaculum of the male type also 

 develops, but no scrotal sacs. The ducts are 

 distinctly of the female type much reduced, 

 and the phallus and mammary glands are defi- 

 nitely female. The general somatic habitus 

 inclines distinctly toward the male side. Male 

 hormones circulating in the blood of an indi- 

 vidual zygotically female have a definitely 

 limited influence, even though the action exists 



from the beginning of morphological sex- 

 differentiation. A detailed study of this prob- 

 lem will be published at a later date. 



Frank E. Lillie 



University op Chicago 



a chemotropic response of the house 

 fly (musca domestica l.)i 



It is generally conceded that the house fly 

 lays its eggs most frequently in fermenting 

 vegetable substances. Of these, fermenting 

 horse manure is most often chosen, and about 

 cities probably ninety per cent, or more of 

 the house flies are bred from this substance.^ 



Although manure varies considerably, de- 

 pending upon the food, the age and the health 

 of the horse, it seems to be invariably attrac- 

 tive to female house flies, provided it is moist 

 and not very old. The flies come to the 

 manure primarily to lay their eggs, and al- 

 though they may obtain some food from it, 

 this is only a secondary object. 



These general observations, together with 

 some preliminary studies recently published,^ 

 led me to believe that the house fly was allured 

 to the manure pile by the odor of some vola- 

 tile chemical substance which was liberated 

 during the early stages of fermentation. Act- 

 ing on this hypothesis, I have tested during 

 the past summer the response of the house 

 fly to a number of inorganic and organic com- 

 pounds which occur as products of fermenta- 

 tion in barnyard manures. 



This paper is a preliminary statement of 

 the results of these experiments. A more de- 

 tailed account will be given in another place. 



Trap Experiments with Ammonia and Other 

 Chemical Substances 

 The following chemical compounds were 

 exposed in glass containers in screen-wire-fly 



1 This work was done in the department of 

 entomology, New Jersey Agricultural College Ex- 

 periment Station, and is published by permissioa 

 of Dr. T. J. Headlee, entomologist of that sta- 

 tion. 



2 Howaxd, L. O., ' ' The House Fly — Disease 

 Carrier," New York, 1911, p. 7. 



3 27th Ann. Bpt. N. J. Agr. College Exp. Sta- 

 tion, 1914, pp. 396-399. 



