THE BIOLOGY OF THE CELL SURFACE 



develop as far as those in which the chromatic substance 

 of the nucleus is drawn into the ectoplasm. 



We conclude therefore that although cleavage and dif- 

 ferentiation normally go hand in hand, the relation is not 

 one of cause and effect. 



The second event enumerated above as occurring during 

 cleavage which has been held as a cause of differentiation, is 

 the appearance of the embryonic axis and of the plane of 

 bilateral symmetry, the so-called median plane. 



For egg-cells the term, polarity, means that with reference 

 to an axis the egg in some way, as arrangement of pigment, 

 location of the nucleus, site of polar body formation, etc., 

 shows structural polar arrangement. Now it is often held 

 that the polar axis of the egg bears some constant relation 

 to the axis or the median plane of the embryo and on this 

 assumption are elaborated theories of differentiation as 

 determined by the egg's polar axis, of a differential axial 

 gradient, and the like. 



In order to clear this problem, we must first distinguish 

 between the polar axis in eggs of radially symmetrical ani- 

 mals and that in eggs of the bilaterally symmetrical with 

 respect to the axis of the embryo in the former eggs and to 

 the median plane in the latter. Although this distinction 

 is easy if only we appreciate certain simple geometrical 

 truths, nevertheless we must give it prominence here 

 inasmuch as so many writers have most unfortunately 

 used axis and plane interchangeably in their theories of 

 differentiation. 



Polar and embryonic axes coincide in eggs whence radially 

 symmetrical embryos arise. A line drawn through the egg 

 from the site of polar body extrusion to the pole opposite 

 is also the gastrular and the embryonic axis. In the eggs 

 of these organisms — Porifera and Coelenterata — the gas- 

 trula forms in such wise that its axis is along traces of the 

 polar axis, no matter by which of the several modes of 

 gastrulation that obtain in these eggs the gastrula arises. 



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