THE BIOLOGY OF THE CELL SURFACE 



In eggs whence develop bilaterally symmetrical embryos 

 does not permit the conclusion that "the axis of the egg 

 shows a definite relation to that of the gastrula, of the later 

 embryo, and of the adult body"; and that "this relation, 

 broadly considered, appears to be constant throughout the 

 Bilateralia."^ In many radially symmetrical eggs that 

 subsequently show bilaterality it is true that a line drawn 

 from the site where the polar bodies lie to the opposite 

 pole is perpendicular to one drawn along the length of the 

 embryo. In others, — e.g., the Qg^ of Amphioxus"^ — the 

 intersection of these lines forms an acute angle. In eggs 

 that are bilaterally symmetrical before fertilization, the 

 medium plane of the embryo lies in the egg's plane of bilat- 

 eral symmetry. 



A survey of embryogenesis in the entire animal kingdom 

 permits the conclusion that the embryo, with the possible 

 exception of the mammalian, arises from the egg-surface. 

 In the present state of our knowledge, the mammalian &gg 

 does not seem to fall in with our generalization, since its 

 surface-cells form trophoblast. In all other eggs embryonic 

 axis or median plane is normal not to the core of the egg 

 but to its surface where the embryo lies. Axis or plane 

 appears, due to new configurations In the protoplasmic 

 system;^ the embryo-axis or the plane of bilaterality is an 

 expression and not the cause of the dIfTerentiation which 

 unfolds Itself as cleavage progresses. 



We turn now to another of the events occurring during 

 cleavage which were enumerated above, the shift of 

 the cytoplasmic inclusions, again seeking the cause of 

 dlflFerentiation. 



' Wilson, 1924. 

 ' Cerfontaine. 



•' One such configuration is undoubtedly set up by the entrance of 

 the spermatozoon into the egg — see Just, 1912. 



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