ORIGINS OF EMBRYONIC PATTERNS 68i 



respective of its own asymmetry or may produce both dextral and sinistral 

 individuals (Crampton, 1916, 1924). Self-fertilized individuals of Ziwwaea 

 peregra or dextral or sinistral pairs give essentially similar results, but 

 some difference of opinion exists as to interpretation in Mendelian terms/s 

 Sturtevant suggested that the data indicate a simple Mendelian case with 

 dextrality dominant but "with the nature of a given individual deter- 

 mined, not by its own constitution, but by that of the unreduced egg 

 from which it arose." Later data, however, seem to require modification 

 of this hypothesis. 



The coincidence of cleavage pattern with the biradial symmetry of the 

 ctenophore has been noted (p. 564); that a biradial pattern is present in 

 the unfertilized egg is maintained by Fischel (1903) and some other au- 

 thors, but it seems possible that the biradial pattern may develop gradu- 

 ally in relation to cleavage and the pre-existing polarity. According to 

 the generally accepted view, the apicobasal axis of the Ascaris egg be- 

 comes the dorsiventral axis of the animal, and the anteroposterior axis 

 becomes evident only after change in position of cells following the second 

 cleavage ; asymmetry in position of certain cells appears later (pp. 570-72) . 

 Except for the morphological polar differential usually visible in the egg 

 and the relation of chromatin diminution to it, nothing is known concern- 

 ing the pattern underlying these developmental events. The data at hand 

 concerning entomostracan Crustacea are particularly confusing. In cer- 

 tain of the forms studied a polar pattern is visible in the egg, but the axis 

 of the first two cleavages is oblique to the axis of this pattern but defi- 

 nitely related to the median plane of the animal. In some species egg 

 polarity and anteroposterior axis coincide; in others — for example, Cy- 

 clops — egg polarity is said to become dorsiventrality of later stages, with 

 basal pole ventral.^'' That the basal pole should be ventral seems rather 

 remarkable in view of the fact that in arthropods generally ventral pre- 

 cedes dorsal in development. Moreover, here, as in Ascaris, the antero- 

 posterior polarity of the animal apparently becomes evident only sec- 

 ondarily, and how it is determined does not appear. 



The full-grown ascidian oocyte gives evidence of polarity in the ec- 

 centric position of the nucleus but no visible indication of cytoplasmic 

 localization, except a surface-interior difference. As described in chapter 

 xiv, a visible dorsiventrality results in the egg of Styela from cytoplasmic 



^5 Boycott and Diver, 1923; Diver, 1925; Boycott, Diver, Hardy, and Turner, 1929; Boy- 

 cott, Diver, Garstang, and Turner, 1930; Sturtevant, 1923; Crabb, 1927. 



^^ See pp. 574-76; also Schleip, 1929, pp. 295-321 and literature cited there. 



