452 M. R. CLARE. 



ceedingly improbable that the independent rates of these deter- 

 minations are due to a secondary source, bacterial for example, 

 contributing in an additive manner to the respiring pupal tissues. 

 If this were the case, we should expect the condition to be 

 indicated by the metabolism curves. 



As applied to Drosophila, the practice of bringing metabolism 

 into relation with pupal weight is warranted only in a most 

 general way. The correlation is most pronounced during the 

 first day of pupal life when the coefficient of correlation for 4-day 

 pupae grown at 25 C. attains a value of 0.47 .071. On the 

 second and subsequent days this value is considerably reduced. 

 Too much importance should not be attached even to this 

 correlation. The relationship often fails completely in individual 

 cases, as is shown in Fig. 5. This fact, taken in connection with 

 the rather frequent tendency toward the establishment of dis- 

 continuous rates, indicates that an understanding of metabolism 

 in Drosophila is not to be sought on the basis of weight of respiring 

 tissue but rather through the impress of factors regarding whose 

 nature we are at present in ignorance. 



A point of considerable interest brought out in Fig. 3 and in 

 other figures for similarly prepared material is the constancy of 

 the relation between the respiratory rates for the individual lots 

 of pupae up to the third day of pupal life. The striking similarity 

 of the metabolism curves for the first and second days of pupal 

 life indicates that the rates for this period are relatively stable 

 and that the establishment of independent rates may occur 

 either before or later but hardly during this period. On the 

 third day, however, after the inauguration of growth and differ- 

 entiation of imaginal tissues, a new order of rates is ushered in, 

 which is that of adult life. As the metabolism curves for the 

 third day in Fig. 3 show, these new rates considered in their 

 entirety still preserve to a slight degree their kinship with those 

 of early pupal life, but each lot of pupae now develops along a 

 new course. It appears that the rates of first-day pupae in no 

 way can serve to forecast those of the flies which will later emerge 

 from them. We have already suggested that the period of 

 institution of new adult rates is a somewhat critical stage in 

 pupal development. The great majority of pupae of arrested 



