104 



CARNEGIE INSTITUTION OF WASHINGTON. 



released their first brood the mean for one individual is 9 minutes 51 seconds. 

 Other data are given in the table. The first-brood mother is approximately 

 44 times as large in actual bulk as a newly released young. Hence, if the 

 rate of carbon-dioxide production were the same, size considered, in young 

 and first-brood mothers, the first-brood mothers should have a reaction time 

 one forty-fourth that for the young, or 21 minutes 19 seconds. Since the 

 actual mean for first-brood mothers is 9 minutes 57 seconds, it is evident that 

 the rate of respiration, and hence the metabolic rate, is greater, size consid- 

 ered, for first-brood mothers than for young females. In fact, it is 219 per 

 cent greater. For fifth-brood mothers (see table) , 84 times as large as the 

 young, the rate is 149 per cent that for the young. For ninth-brood mothers, 

 approximately 121 times the bulk of the young, the rate is 66 per cent that 

 of the young. The table gives figures for these comparisons. 



Data on the rate of carbon-dioxide production in developmental stages of Simocephalus 



exspinosus. 



"These data indicate that metabolic rate is relatively high in the first- 

 brood mothers, decreases in the fifth-brood mothers, and shows a still 

 greater decrease in the already senescent ninth-brood mothers. The mature 

 males may be fairly compared with the first-brood mothers. Their rate 

 is, bulk considered, approximately 111 per cent that for the females at the 

 same stage of development. The data are consistent with data of other 

 workers for higher (and much larger) animals, including man, in which the 

 metabolic rate decreases later in life in both sexes and is higher for the 

 male. This first attempt to obtain a measure of metabolic rate in Clado- 

 cera opens up a large field, and it is hoped will be extended and continued to 

 the sex-intergrade stock and to several other Cladocera problems." 



Selection with Sex-Intergrade Strain of Daphnia longispina. 



Experiments on the effect of selection upon the degree of sex intergradeness 

 in various strains of Daphnia longispina (referred to in previous Year Books) 

 have been completed. Sex intergradeness appears due to a factor (or factors) 

 which is subject to genetic changes, so that selection and return selection are 

 effective, success depending of course upon our utilizing mutants in our selec- 

 tions. Intergrade production in the lines selected toward reduction of the 

 character has in some cases become restricted to the scanty production of 

 slightly intergrade individuals, and in one line no intergrades of any sort have 

 appeared for eight successive generations. On the other hand, the high 

 trains produce mostly intergrades of fairly high rank. 



