38 AN INTRODUCTION TO MODERN GENETICS 



6. Chromosomes and Factors 



In order to show that the chromosomes behave like Mendelian 

 factors we must demonstrate (i) that they have an individuahty, that is 

 to say that each one produces a specific effect on development and thus 

 on the kind of adult which is produced; (2) that they retain this indivi- 

 duahty throughout many cell divisions; and (3) that in a hybrid they 

 segregate Hke Mendelian factors. 



6a. Evidence that the chromosomes have individual characters 



The classical evidence that the chromosomes have qualitatively 

 different effects on development is that of Boveri (1907). By fertilizing 



Fig. U. Diagram of Triaster and Tetraster Eggs in Echinoderms. — In A a 



diploid number of chromosomes is being divided to give three daughter nuclei, 

 in 6 a triploid number giving four nuclei. 



(After Boveri.) 



sea urchin eggs with an excess of sperm, he obtained double fertiliza- 

 tion (two sperm entering one egg) which resulted in the formation of 

 tetra-polar asters; and by shaking normal monospermic eggs he got 

 triasters. The first four cells formed in the tetraster eggs, or the first 

 three in the triaster eggs, could be separated by using Ca-free sea water. 

 The percentage of the isolated cells which developed normally to the 

 pluteus stage agreed approximately with the probability that the cell 

 contained at least one of each kind of chromosome, remembering that 

 in the first case 3« chromosomes had to divide and be distributed 

 among four cells and in the second case 2« chromosomes among three 

 cells. Moreover, Boveri could show that mere variation in number of 

 chromosomes was not in itself enough to stop development, which was 

 possible with either the haploid number (one of each kind), or with 



