ORIGIN OF POLARITY, SYMMETRY, AND ASYMMETRY 69 



discovered even in the egg of sea-urchins. If the egg is exposed to 

 certain anaerobic conditions, a pit is formed in a particular place, 

 but disappears on returning the egg to normal conditions. By means 

 of ifitra vitam stains, it has been shown that the site of the pit 

 coincides with the ventral side of the future larva.^ 



At the same time, as in the case of the amphibian tgg, this labile 

 determination of the plane of symmetry can be overriden by a 

 variety of factors, of which, however, the point of sperm-entry is 

 not one. 2 Artificial stretching and deformation of the tgg (in a 

 direction making some fairly large angle with the axis of polarity) 

 leads to the determination of the dorso-ventral axis along that of 

 tension. The primary axis of polarity is unaffected.^ 



Artificial rearrangement of the egg-contents has also been shown 

 to influence the localisation of the plane of bilateral symmetry. In 

 the sea-urchin, Psammechmtis miliaris, the presence in the ripe egg of 

 a subcortical layer of lipoid granules has been observed* and they 

 may be displaced by means of the centrifuge. Neglecting those 

 cases in which the granules are heaped up at either the animal or 

 vegetative poles, it is found that the meridian of the egg on which 

 the granules are accumulated becomes the ventral side of the larva. ^' ^ 

 Similarly, the visible granules of the egg of Arbacia can be con- 

 centrated on any meridian, which then becomes the ventral side of 

 the larva^ (see also p. 218). The dorso-ventral axis, it seems, is 

 determined as a gradient with high point ventrally. The curious 

 fact that the ventral surface is associated in Psammechiniis with 

 centripetal lipoid granules, but in Arbacia with centrifugal 

 yolk-particles, can be explained if their concentration leads to 

 relatively higher metabolism. Similarly the ends of the stretched 

 egg appear to be in a peculiar labile active condition. Interesting 

 possibilities of analysis are here opened up. 



In some Echinoderms, the dorso-ventral axis is visible in the 

 unfertilised egg (Psoitis phantappusy and marked by a particular 

 distribution of yolk, or (Asterma gibbosa)^ by an elongation of the 

 egg. The latter state of afl^airs is also found in some insects and 



^ Orstrom, in the press. 2 Horstadius, 1928. 



^ Boveri, 1901 ; Lindahl, 1933 a. ^ Runnstrom, 1924. 



^ Runnstrom, 1925. ^ Lindahl, 1933 b. 



' Runnstrom and Runnstrom, 1921. ^ Horstadius, 1925. 



