MARY J. (il'THRIE AND HOPE HIBBARD. 



arises earliest, with very few exceptions, in those groups of 

 echinoderms in which the- larva' have their skeleton earliest 

 developed." Since one ot the functions ot the primary mesen- 

 chyme is the formation of the skeletal rods of the larva* the 

 reason for this correlation is evident. 



In this paper it has been shown that the primary mesenchyme 

 in Toxopneustes comes from the micromeres. Driesch (1900) 

 has shown by experiments on fragments of blastuhe, that after 

 the micromeres have been formed, the cells at the opposite pole 

 of the larva are unable to form mesenchyme. The micromeres 

 arise at the vegetal pole of the segmenting egg, but cannot be 

 considered as either ectoderm or cndoderm. The primary mesen- 

 chyme is the result of an earlier differentiation than either 

 ectoderm or endoderm. 



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