176 ILLINOIS STATE ACADEMY OF SCIENCE 



THE EFFECT OF SELECTION ON THE LENGTH 

 OF SPINE IN DAPHNIA LONGISPINA 



Mrs. Margaret Smith Young, Chicago 



The work was started Dec. 27, 1922, and so far there 

 have heen ten generations. This paper is, therefore, 

 merely a preliminary one. 



The Daphnia stock was obtained from the Laboratory 

 of the University of Chicago where the work was carried 

 on. The stock came originally from a fish fancier and 

 had been kept in the laboratory for about a year before 

 this experiment was begun. 



A parthenogenetic female (3027) was selected to start 

 the first (3028) 1 generation. A brood is produced about 

 every other day. The stock is kept in fingerbowls under 

 greenhouse conditions, and temperature, food and other 

 environmental conditions kept as nearly uniform as pos- 

 sible. The food consists of a coccus (Chlamyda monas) 

 which causes the water to look green. Successive broods 

 are termed A, B and C, and broods A, B and C are used 

 as a basis for all the mathematical calculation. From 

 brood A of 3028 1 an individual was selected to start the 

 minus strain, and another to start the plus strain of 

 3029 2 . The difference in length of spine between the two 

 was not great and was taken without measuring. After 

 this the length of body, divided by the length of spine and 

 called " Index", was made the basis of all calculations. 

 The plus or long spine strain will therefore have a smal- 

 ler index than the minus or short spine strain. 



Selection was made from the A brood of ±3029* for 

 the ±3030 3 generation, the indices being respectively .3 

 and .4. 



In order to have better proof of the hereditary quality 

 of the character, the A broods of ±3030 3 were measured 

 alive and the five most favorable • animals respectively 

 were used to start five plus lines and five minus lines of 

 3031*. It was then thought best to use the method of se- 

 lecting in each generation five individuals of the plus 

 strain and five from the minus strain of the B brood be- 

 longing to the most favorable A brood. This was done 

 to start generation 3032 5 . The curves of this generation 



