672 



SCIENCE. 



[N. S. Vol. XVI. No. 408. 



and in some cases the colonies obtained by 

 plating on litmus-lactose-agar represent an 

 almost pure growth of B. coli. If, instead of 

 plating after the short period of growth, the 

 original culture is allowed to develop for 

 twenty-four, or even for eighteen, hours, B. 

 coli is isolated only with much greater 

 difficulty. 



The explanation of these facts is apparently 

 simple. In the first few hours a rapid develop- 

 ment of colon bacilli occurs, while other micro- 

 organisms present multiply more slowly, but 

 if a longer incubation period is allowed, the 

 other microorganisms, especially the strepto- 

 cocci recently described in Science by Mr. C. 

 E. A. Winslow and Miss Hunnewell, develop 

 abundantly and overgrow the colon bacilli. 

 This over-growth is jjrobably to be explained 

 by a study of the products of the two kinds of 

 microorganisms. The colon bacilli produce 

 lactic acid, but also under favorable condi- 

 tions carry on putrefactive processes with the 

 ultimate formation of alkaline matters which 

 partially or entirely neutralize the acid 

 formed. The streptococci flourish only in the 

 presence of sugars, but produce abundant acid 

 and, while, therefore, perhaps growing more 

 slowly at the start, eventually produce much 

 more lactic acid than does B. coli. Moreover, 

 colon bacilli appear to be extremely sensitive 

 to lactic acid of some strength and are there- 

 fore inhibited, if not actually killed, by the 

 acid produced by the streptococci. 



The method of procedure here otitlined has 

 given satisfactory results not only in the Insti- 

 tute laboratories, but also at the hands of 

 other investigators than ourselves who at my 

 request have kindly tested it. 



S. C. Peescott. 



Biological Laboratoeies, 

 Massachusetts Institute of Technology. 



the eggs of mosquitoes of the genus culex. 

 The conventional description of the oviposi- 

 tion of Culex has been rendered obsolete by 

 recent observations. This description was 

 based on the eggs of Culex pipiens Linn., a 

 species which deposits them in large, boat-like 

 masses, floating on the surface of water. The 

 eggs do not hibernate. This has been assumed 



to be the general manner of oviposition in the 

 genus Culex, but such is not the ease. So far 

 as at present known, Culex pipiens is the only 

 Culex that so deposits its eggs. 



The species of Culex may be divided into 

 two groups, the first comprising those species 

 in which the legs are unhanded, the second 

 those in which the tarsal joints are banded 

 with white rings. The method of egg laying 

 is different in these groups. In the first group, 

 the eggs are generally laid floating on water 

 and apparently they do not hibernate. Culex 

 pipiens belongs to this group, but its boat- 

 shaped masses of eggs represent the extreme 

 form of development of the floating type of 

 egg. In Culex melanurus Coq., the eggs are 

 laid singly, floating on the surface of water; 

 in C. territans Walk., they are laid in little 

 groups of two or three, side by side and also 

 floating; finally, in C. pipiens Linn, we have 

 the well-known boat-shaped mass. However 

 this type is not exclusive for the dark-legged 

 species of Culex, for C. iriseriatus Say lays its 

 eggs singly, adhering firmly to objects at the 

 extreme edge of the water, and the species 

 doubtless hibernates in this state. 



In the ring-legged species of Culex, the gen- 

 eral type of egg described by Professor John 



B. Smith (Science, N. S., XV., 391, 1902) 

 obtains. C. sollicitans Walk., described by 

 Professor Smith, laid its eggs dry at the edges 

 of places where water was likely to collect 

 and the larvae hatched when water appeared. 



C. canadensis Theob. lays its eggs singly and 

 they do not float on the water, but mostly sink 

 to the botto'm. In this species the eggs will 

 hatch in part in the presence of water, but 

 most of them remain unhatched till the winter 

 is passed. There seems to be a full brood of 

 these mosquitoes early in spring from hiber- 

 nated eggs, after which only scattering eggs 

 hatch, most of them going over to the next 

 season, whether wet or dry. In C. sylvesiris 

 Theob., the eggs are laid similarly and sink in 

 water, but the species breeds continuously all 

 the summer, practically all the eggs hatching 

 when covered by rain water. But a set of eggs 

 obtained in September all hibernated, although 

 they were kept wet. 



There remain many species of Culex whose 



