i66 



CYTOLOGY 



CHAF. 



fact that the first three nuclei of a triaster have a far greater chance of 

 each getting a representative of all the chromosomes of the series than 

 have the four nuclei of the tetraster. 



B 



D 



Fig. 76. 



-^-^|5*?< 



Development of dispermic Echinoderm egg=. (After Boveri, ZelUn-Studien, 1907.) A, blastula of the 

 tetraster type (Echinus). About one quarter of the wall of the blastula is'falling into separate cells; B, C, 

 D, plutei from triaster eggs (Strongylocentrotus). B, with a normal skeleton on the left side, rudimentary on 

 the right ; C, with skeleton present on left side only ; D, a perfect pluteus. The dotted lines indicate the 

 boundaries of the regions derived from the three primary blastomeres. Note the difierence in the size of the 

 nuclei in the different regions. 



Boveri also reared tetraster and triaster dispermic eggs as whole 

 embryos {i.e. without separating the first four or three blastomeres 



