No. I.] THE EMBRYOLOGY OF CREPIDULA. 175 



the first few divisions. Wilson ('92 and '93) cites Amphioxus, 

 Echinus, Synapta, Antedon, and Sycandra as typical examples 

 of the orthoradial type of cleavage. In his work on Amphioxus, 

 however, he says that the radial form is present in only about 

 three-quarters of all the eggs of that animal, and that even in 

 these the " cross furrow " may or may not be present. The 

 presence of a polar or cross furrow is in itself sufficient evidence 

 that the cleavage is not strictly orthoradial. Among the 

 echinoderms I have observed that polar furrows are usually 

 present in the eggs of Asterias and Arbacea, and I believe that 

 some of the cases in which orthoradial cleavage is figured may 

 be attributed to the influence of the usual teachings upon this 

 subject. But granting that there are some eggs, as appears 

 to be true, in which the cleavage is orthoradial up to the 8- 

 or i6-cell stage, there is certainly no egg which preserves the 

 orthoradial condition through any considerable part of the 

 cleavage. This form of cleavage is not only very rare, but 

 when found it is exceedingly evanescent and very soon gives 

 way to spiral cleavages. Driesch ('92) has well said : " Das so 

 oft schematisch gezeichnete Vierzellenstadium mit zwei sich in 

 zwei Puncten schneidenden Meridianen kann man wohl getrost 

 aus der Reihe des Existirenden streichen; vgl. hierzu die 

 genauen Furchungsstudien von Chabry und Rauber. Das 

 Princip der kleinsten Flachen, dessen nothwendiger Ausdruck 

 (Plateau, Lamarle) es ist, dass stets drei Flachen in einer 

 Kante, vier Kanten in einem Punkt zusammenstossen, scheint 

 gerade in der Ontogenie der Thiere besonders deutlich zu Tage 

 zu treten." 



(2) Spiral Cleavage. — The word "spiral" has long been 

 used (Selenka '81, Lang '84) to describe that form of cleav- 

 age in which there is an actual or virtual rotation of the 

 blastomeres upon each other. Wilson ('92), however, first 

 proposed the recognition of a spiral type of cleavage. "The 

 spiral type," he says, "arises from the radial through a twist- 

 ing of the radii, as it were, the blastomeres being displaced 

 or rotated, with respect to the egg axis, either to the right, 

 following the hands of a watch (right-handed spiral), or in 

 the reverse direction (left-handed spiral), as the case may be. 



