BEAHEES OF HEREDITARY UNITS 217 



loss of tlie paternal chromosomes, especially in the light 

 of the reciprocal cross. 



In this reciprocal cross, the egg olf SphcRrechmiws 

 is fertilized by the sperm of Strongylocentrotus. All 

 the chromosomes of the segmentation nucleus divide 

 and pass regularly to the two poles (Fig. 99, fe). The 

 hybrid embryo shows characters of both parental species. 



O 





o 



1- 9.0^08 

 vo o oo 



/'o ' oo\, 



I 00 



\0n O 



0. 







o 



/O ^ ; 



io 0/ 



Fig 101. — Larval aea urchin seen in side view. On one side it allows hybrid characters, 

 on the other side it is maternal. The sizes of the nuclei on these two sides, as seen in the 

 figure, coincide with the view that the hybrid side is diploid and the maternal side haploid. 

 After Herbst.) 



The difference in the two cases can be safely attributed 

 to the observed differences in the fate of the chromosomes, 

 rather than to unrecognized differences in other elements 

 brought in by the sperms. 



Herbst 's experiments contribute further evidence in 

 favor of the chromosome interpretation. He caused the 

 unfertilized eggs of a sea urchin to begin to develop 

 parthenogenetically by adding a little acid to the sea 

 water. After five minutes the eggs were removed to pure 

 sea water, and sperm of another species, Strongylocen- 



