ORIGINS OF EMBRYONIC PATTERNS 677 



and interior of the blastoderm; this is perhaps also true for the mammals. 

 Differential dye reduction in Drosophila ovaries suggests that in this 

 form ventrodorsahty of the egg may be determined by a differential 

 between surface and interior of the ovary itself (p. 144). 



SYMMETRIES AND ASYMMETRIES IN ECmNODERMS 



Echinoderm development presents perhaps the most remarkable se- 

 quence of symmetries and asymmetries of any animal group. The early 

 embryonic stages of most echinoderms appear to be completely radial, 

 though ventrodorsahty is indicated in some species in early cleavage or 

 even before cleavage.^* Various lines of experiment also give evidence of 

 a ventrodorsal gradient in early stages and of a right-left gradient in 

 the apical archenteron.'^ Development of defective skeletons in plutei 

 from isolated 1/2 blastomeres led Plough (1927, 1929) to postulate locali- 

 zation at or before first cleavage, of skeleton-forming material in the basal 

 part of the egg, excentric in the ventrodorsal axis and bilateral. However, 

 according to W. Marx (193 1), skeletal defects of larvae from early blasto- 

 meres result from ectodermal conditions. An ectodermal bilaterality, pres- 

 ent at or before the two-cell stage, consists of two regions conceived es- 

 sentially as gradient systems, decreasing from centers of highest potency. 

 These, rather than localization of skeleton-forming material, determine 

 bilateral aggregation of mesenchyme and so the skeletal bilaterality; in 

 partial forms from isolated blastomeres this bilaterality is defective. This 

 conclusion is in accord with earlier work on the relation of skeletal locali- 

 zation to ectoderm. Skeleton-forming material is localized in the most 

 basal region of the unfertilized Arbacia egg, according to Horstadius 

 (1937a), not between the nucleus and center of the egg (Harnly, 1926). 



The question of the relation of first and second cleavage planes to the 

 median plane of the larva has been investigated in a number of echino- 

 derms by intravital staining of one cell of the two-cell stage with Nile blue 

 sulphate.^" This procedure shows, in Echinus, Par echinus, and Paracen- 

 trotus, 50-59 per cent coincidence of first-cleavage plane and median plane; 

 17-42 per cent with first cleavage plane frontal, that is, at right angles to 

 the median plane; and the remainder mostly oblique, with small percen- 

 tages uncertain. In the clypeastroid Echinocyamus the first plane is usu- 

 ally frontal. In the asteroid Astropecten it is usually median (41. i per 



'* J. Runnstrom, 1920; J. und S. Runnstrom, 1920. 



■' See pp. 134-36, 219-21 . 



" Von Ubisch, 1925a; Runnstrom, 19266; Horstadius, 192S6. 



