THE CHROMOSOMES 



147 



species of sea urchins. The best analyzed cases are 

 those that Baltzer has worked out. Grosses were 

 made between four species of sea urchins; one such 

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

 of Sphserechinus were fertilized by the sperm of 

 Strongylocentrotus. The division of the chromo- 

 somes proceeded in normal manner. The pluteus 





:vk. 







I 



-^^;V'^: 







..v'-''"' ■ 



'■'III IIIHI^ 

 « 



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

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

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

 locentrotus; 4 and 4o, same, equatorial plates; .5aiid5f;,hyl)rid of Strongy- 

 locentrotus by Spha^rechinus 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 reci]:)rocal cross was made 

 by fertilizing the eggs of Strongylocentrotus with the 

 sperm of SphaTcchinus. At the first cleavage of the 

 egg some of the chromosomes divide normally, wliik^ 

 other chromosomes remain inactive and finally be- 



