ORIGINS OF EMBRYONIC PATTERNS 679 



tern of these is primarily asymmetrical and becomes radial only seconda- 

 rily. 



The oral-aboral polarity of the adult echinoid, asteroid, and ophiurid 

 is a new polarity, not coincident with that of the egg and embryo. In 

 holothurians the original polarity persists as the longitudinal axis of the 

 adult, and in the crinoids there is apparently a complete reversal of polar- 

 ity in metamorphosis. In addition to these changes in pattern, the radial 

 pattern of the adult becomes an anteroposterior polar pattern in clypeas- 

 troids and spatangoids; an anteroposterior motor pattern is present in 

 different degree in some other forms; and in certain holothurians the 

 radial pattern becomes ventrodorsal, that is, certain radii become differ- 

 ent from others and function as ventral side. If the various echinoderm 

 patterns originate as gradient patterns in relation either to the ovarian 

 environment of the oocyte or to gradient factors already present, it is 

 evident that evolution of spatial pattern in the group has involved changes 

 in the gradients. In this connection the approach of asteroid to echinoid 

 larval pattern with experimental differential inhibition and of echinoid 

 to asteroid pattern with secondary modifications of differential recovery 

 may be recalled (pp. 208, 217). 



SYMMETRIES, ASYMMETRIES, AND DETERMINATE CLEAVAGE 



It was pointed out in chapter xiv that in forms with spiral cleavage the 

 relation between the first two cleavages and the median plane is appar- 

 ently constant for the species but that different authors do not agree as 

 regards the relation in different forms. However, the species differences 

 in size of somatoblasts, lack of coincidence of the median planes in differ- 

 ent cells, and shifts in position of cells make it uncertain whether the 

 apparent differences are really significant. Dorsiventrality, as indicated 

 by difference in cell size, may be present or absent in early cleavage pat- 

 tern quite independently of symmetry or asymmetry of development. It 

 is not even certain whether a pattern of dorsiventrality or asymmetry 

 is always present before fertihzation or before cleavage. That the full- 

 grown oocytes of these forms possess a spatial pattern of some sort, at 

 least a polar pattern, appears beyond question; but this pattern may be 

 distinguishable only in somewhat eccentric position of the nucleus toward 

 the apical pole. Beginning with the breakdown of the oocyte nucleus, or 

 later with polar-body formation, or still later on with fertihzation, changes 

 in visible pattern very commonly occur, consisting in localization, usually 

 in definite relation to the polar axis, of visibly distinguishable cytoplasmic 



