ly-t TOR RE Y 



minate cleavage there is more than one method of procedure 

 by which a practically identical result may be attained. 



Finally, I would call attention again to the frequency of the 

 cases in which radial cleavage and symmetry is retained after 

 bilateral symmetry should have expressed itself. Bearing this 

 in mind it would seem that Heath ('99) is correct in his criti- 

 cism of Conklin's ('97) conclusion that '' radial structures in 

 the developing egg are a foreshadowing of larval characters, 

 just as bilateral cleavages are usually attributed to a precocious 

 development of adult characters." In reference to this state- 

 ment Heath says : " The reflection of larval stages on early 

 cleavage stages does not produce radial symmetry but tends to 

 destroy it." These early radial characteristics may be better ex- 

 plained, as Heath further points out, by considering them as 

 having been inherited from the radially symmetrical ancestors 

 of the Tiirbellaria^ annelids and molluscs. The transition from 

 the equal to the unequal type of cleavage has been, without 

 doubt, very gradual. With the shortening of the larval exist- 

 ence the posterior macromere has gained in importance until 

 finally we reach the direct teloblastic development of oligo- 

 ch^etes and leeches. 



