678 PATTERNS AND PROBLEMS OF DEVELOPMENT 



cent) or frontal (44.6 per cent), rarely oblique (3.6 per cent), and some- 

 times equatorial (10.7 per cent). In Asterias it is frontal. The eggs of the 

 holothurians Cucumaria and Psolus possess a distinguishable ventrodorsal 

 pattern and are ventrodorsally elongated; the lirst cleavage plane is 

 frontal and in later cleavages a ventrodorsal difference in cell size appears 

 (J. und S. Runnstrom, 1920). These data suggest a difference in degree of 

 development of ventrodorsality at the time of early cleavage in different 

 echinoderms and perhaps also in different individuals of the same species. 

 In the regular sea urchins investigated ventrodorsal pattern apparently 

 usually prevents obliquity of the first two planes, and the first plane is 

 more often median than frontal; but in the holothurians the ventrodorsal 

 pattern, or perhaps merely the ventrodorsal elongation of the egg, deter- 

 mines orientation of the first cleavage spindle in the long axis, and a 

 frontal cleavage plane results. In general, both observation and experi- 

 ment seem to indicate that ventrodorsality develops gradually, becoming 

 an effective factor earlier in some species than in others. That ventro- 

 dorsality of the sea-urchin egg might be determined at fertilization, the 

 meridian of sperm entrance becoming median ventral, was suggested by 

 Jenkinson (191 1&). However, if sperm enters through the micropyle at 

 the apical pole, there is no such meridian. The physiological basis of ven- 

 trodorsal pattern is apparently present in the unfertilized eggs of some, 

 perhaps of all, echinoderms; but conclusive evidence as to manner and 

 time of its origin is lacking. From the blastula or early gastrula stage 

 the larvae of sea urchins and starfishes are bilateral in appearance and 

 show a distinct physiological ventrodorsality (pp. 134-39). 



The lateral asymmetry of the hydrocoel becomes evident at different 

 developmental stages of the coelom. In some species earlier stages of the 

 coelom sacs appear alike on the two sides, even to formation of two 

 hydrocoel primordia, but one degenerates later. In others asymmetry be- 

 comes evident early in coelom development. The usual position of the hy- 

 drocoel on the left side, though readily altered experimentally, indicates 

 that genetic factors are in some way involved in the laterality; but how 

 the asymmetry originates and how the definite spatial relation to ventro- 

 dorsality and to polarity are determined, remain to be discovered. The 

 radial features of the adult echinoderm develop from the lateral asym- 

 metry. The left coelom, a part of which usually becomes the hydrocoel, 

 shows no evidence in its earlier stages of the later radial pattern. De- 

 velopment of the ambulacral canals from the hydrocoel probably involves 

 spatial relations of dominance, like tentacle development; but the pat- 



