114 



THE CHROMOSOMES 



species of sea urchins. The best analyzed cases are 

 those that Baltzer has worked out. Crosses were 

 made between four species of sea urchins; one such 

 cross will serve as an example (Fig. 39). The eggs 

 of Sphaerechinus were fertilized by the sperm of 

 Strongylocentrotus. The division of the chromo- 

 somes proceeded in normal manner. The pluteus 



FIG. 39. 1 and la, chromosomes in the first normal cleavage spindle of 

 Sphserechinus; 2, equatorial plate of two-cell stage of same; 3 and 3a, 

 spindles of two-cell stage of egg of hybrid of Sphaerechinus by Strongy- 

 locentrotus ; 4 and 4a, same, equatorial plates ; 5 and 5a, hybrid of Strongy- 

 locentrotus by Sphaerechinus cleavage spindle in telophase ; 6, next stage 

 of last; 7, same, two-cell stage; 8, same, later; 9, same, four-cell stage; 

 10, same, equatorial plate in two-cell stage (22 chromosomes); 11, same, 

 from later stage, 24 chromosomes. (After Baltzer.) 



that developed was intermediate in character; or at 

 least showed peculiarities both of the maternal and 

 of the paternal types. The reciprocal cross was made 

 by fertilizing the eggs of Strongylocentrotus with the 

 sperm of Sphaerechinus. At the first cleavage of the 

 egg some of the chromosomes divide normally, while 

 other chromosomes remain inactive and finally be- 



