August 20, 1915] 



SCIENCE 



255 



developed in the region of his chest, where the 

 flies had gained access. This suggests Ohry- 

 sops, the members of which are commonly 

 called deer-flies, and it is extremely likely that 

 this genus may act as egg-carrier for Derma- 

 tohia quite as frequently as do mosquitoes. It 

 should be stated that the Dermatobia flies are 

 not yellow, but of a dark metallic green. 



As to the exact modus operandi in the case 

 of Dermatobia, it is quite certain that oviposi- 

 tion on foliage is not practised, but that the 

 fly captures the elected carrier and holds it 

 while gluing the eggs firmly by their caudal 

 end to the underside of its body, leaving the 

 cephalic end of the eggs free and in such posi- 

 tion that it will come in immediate contact 

 with the skin of any animal bitten by the 

 carrier. Once the carrier alights on a warm- 

 blooded animal, the heat of the latter's body 

 causes the maggot to spring the lid of the 

 chorion and to work its way immediately and 

 doubtlessly very rapidly into the skin, most 

 probably by way of a hair follicle. As sug- 

 gested by Mr. Crawford, it is practically 

 certain that the empty chorion remains at- 

 tached to the carrier after the exit of the 

 maggot. Chaeles H. T. Townsend 



TJ. S. National Museum, 

 July 3, 1915 



RAPID METHOD OP COUNTING BACTERIA IN MILK'- 



The satisfactory control of milk supplies 

 would be facilitated by a rapid method of de- 

 termining the bacterial content. There can 

 be no question but what the most accurate 

 count is obtained by incubating plate cultures 

 for five days at room temperature, but, in 

 spite of this, two days at 37° 0. is the only 

 standard method. This has been adopted be- 

 cause of the urgent demand for a quick an- 

 swer. Because of the advantage of obtaining 

 results rapidly, the direct microscopical ex- 

 amination of milk is frequently urged. In 

 spite of the obvious weaknesses of this method, 

 such as the errors in measuring the small 

 quantities needed or in centrifuging, and the 

 fact that dead bacteria can not be differen- 



1 Preliminary note. Publication authorized by 

 the Director of the Wisconsin Experiment Station. 



tiated from the living, this method has its 

 earnest advocates. 



If it were possible to use easily measurable 

 quantities of milk, i. e., from Mo to 1 cc, and 

 grow the germs contained therein so that only 

 those capable of forming colonies would be 

 counted, and if this count could be obtained 

 within, say, six hours, the demands in the case 

 would be reasonably met. 



If such an accurate count could be obtained 

 in a few hours, it would be possible for the 

 producer or dealer to determine actually the 

 bacterial content of his product before putting 

 it on the market. This would also enable the 

 health ofiicial to hold a sample of milk sus- 

 pected of being beyond the limits permitted 

 until the count could be actually obtained, 

 when the samples in question could be either 

 passed or justly condemned. Under present 

 conditions, when the bacteria are determined 

 by ordinary cultural procedures, such a course 

 is out of the question because it is not pos- 

 sible under any conditions to obtain a count 

 in less than forty-eight hours. 



It is possible now to suggest a rapid method, 

 which,' I believe, will meet any reasonable de- 

 mands. The method is a combination of the 

 direct count and the culture methods and is 

 obtained by making small plate cultures on a 

 microscopic slide. These little plates are incu- 

 bated for several hours (three to six), then 

 the medium is dried down and stained so as 

 to bring out in sharp relief, when examined 

 under the microscope, the minute colonies 

 which have developed. 



It is not proposed to go into definite details 

 in this preliminary paper^ but rather to define 

 the lines along which the investigation is pro- 

 ceeding. In explanation of the methods, how- 

 ever, it may be said that about one tenth of a 

 cubic centimeter of mills; is mixed with stand- 

 ard agar and spread over a definite area of a 

 sterile glass slide. When the agar is hard, 

 this little plate culture is put in the incubator 

 for about six hours, under conditions which 

 prevent evaporation. It is then dried, given 



2 An extended account of the method and the re- 

 sults obtained in a series of analyses will soon ap- 

 pear. 



