672 PATTERNS AND PROBLEMS OF DEVELOPMENT 



In other forms of development polarity is determined by various ex- 

 ternal differentials, and polarity determined by one differential may be 

 obliterated and a new polarity determined by another in many animals. 

 In some cases of unicellular development polar pattern is evidently de- 

 termined by direction of the last preceding mitosis. It is possible that 

 every mitosis may determine some degree of cell polarity, perhaps evanes- 

 cent, or obliterated by the next mitosis in a different direction or by a 

 differential of one kind or another — for example, the general physiological 

 gradient and dominance in a multicellular organism, a differential in 

 oxygen supply, electric potential, light, etc. In the absence of an effective 

 differential external to the cell the polarity determined by the preceding 

 mitosis may perhaps persist and become a definitive axiate pattern. Per- 

 haps the polarities of some eggs are initiated at the final oogonial division. 

 In other eggs a polarity so determined may be obliterated by factors in 

 the intraorganismic environment. In the egg of Fucus the original polar- 

 ity, however determined, is evidently readily obliterated by various ex- 

 ternal factors (pp. 423-25). At present there seems to be no adequate 

 ground for concluding that polarity of the egg is different in origin and 

 nature from other physiological polarities of cells and multicellular sys- 

 tems. 



SYMMETRIES AND ASYMMETRIES OF EMBRYONIC DEVELOPMENT 



The features of developmental pattern commonly distinguished as sym- 

 metries and asymmetries from polarity have often been regarded as polari- 

 ties in other directions than the chief or primary polarity. They appear 

 in definite relations, characteristic for the species, to the polar, major, 

 or primary axiate pattern. Organismic pattern may be radial about the 

 polar axis, ventrodorsal or dorsiventral with a resulting bilateral pat- 

 tern at right angles to the polarity; or asymmetries — lateral, spiral, or 

 irregular and specific — may be present. In many animals a polarity is evi- 

 dent at or before the beginning of embryonic development; but ventro- 

 dorsality, dorsiventrality, or asymmetry becomes evident only at some 

 later developmental stage and perhaps gradually. The question how and 

 when the symmetries or asymmetries originate and what their physio- 

 logical basis is, has been and still is the subject of much discussion and 

 speculation. Experimental evidence indicates that the basis of symmetries 

 and asymmetries of the earlier developmental stages of various animals 

 is present in the\egg, apparently even before fertilization, though the 

 patterns mayjbe experimentally alterable. The point of entrance of the 



