THE CHKOMOSOMES 143 



Haans, Herbst, and Boveri on giant eggs of sea 

 urchins fertilized b}^ sperm of another species. The 

 hybrid larvae produced when normal eggs of one 

 species are fertilized b}' sperm of the other species 

 are intermediate in character between the two 

 parental types of larvae; while those from giant eggs 

 of the same species fertilized bj' sperm of the other, 

 also intermediate, incline more to the maternal side. 

 The nucleus of the giant egg is double the size of 

 that of the normal egg and according to Bierens de 

 Haans the chromosomes are also double in number. 

 Consequently, the amount of maternal chromatin 

 should be double that introduced by the sperm, and 

 might produce a corresponding influence on the 

 hybrid character. But since in these giant eggs the 

 cytoplasm is also doubled, it is not evident that the 

 results are due to the chromosomes rather than to the 

 cytoplasm. By means of the following ingenious 

 comparison Boveri has shown that the results must 

 be ascribed to the chromosomes rather than to 

 cytoplasm. Normal eggs were broken into frag- 

 ments, the nucleated pieces were fertilized with the 

 sperm of the other species, and those fragments of 

 half the volume of the normal egg were isolated. 

 As is known, such fragments develop into whole 

 larvae, whose nuclei will have the usual chromatin 

 content. The egg cytoplasm is, however, reduced to 

 half. Nevertheless the larvae did not incline to the 

 paternal side, although these larvae, like all larvae 

 from fragments, were often simi)ler than the normal. 

 Hence since a relative decrease in the amount of 



