Figure 53. Results of crossing different genera of sea urchins. A and 

 B are the larval skeletons of the parents, A, Paracentrotus, B, 

 Sphaerechinus. 



C, Normal hybrid, intermediate between the two parents. D, 

 Hybrid that is entirely maternal in type, resulting from the failure 

 of the sperm chromosomes to take part in development. After Herbst 

 (1925). 



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