398 BERTRAM G. SMITH 



8. In the late blastula, bilateral symmetry is manifested by both 

 the superficial cleavage pattern and the internal structure, and 

 this condition is undoubtedly an expression of the definitive 

 bilateral symmetry of the embryo. 



9. The bilateral symmetry of the late blastula is the conse- 

 quence of dorsoventral differentiation imposed upon the pre- 

 existing radial symmetry and anteroposterior differentiation of 

 the egg. 



10. The subsequent development of the two lateral halves of 

 the egg is conditioned and controlled by the differentiation 

 which proceeds along the two axes (anteroposterior and dorso- 

 ventral) already established; these two axes sufF.ce to determine 

 and maintain the bilateral symmetry of the embryo. 



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