Asymmetry of Larval Amphioxus

Abstract

The evidence for the theory that Amphioxus is “essentially a pædomorphic transformation of a sessile Protascidian” is even stronger to-day than when Garstang¹ first proposed it. The more recent of Conklin's investigations² have shown the early development of Amphioxus to be “more strikingly like that of Ascidians than has been recognized hitherto”, and the fate-map of Amphioxus is very closely similar to that of Ascidiella³. One apparent anomaly—that the egg of Amphioxus, unlike the ascidian's, is still of the ‘regulation’ type after its first cleavage²—yields to Berrill's quantitative proof⁴ of the relative retardation of the development of Amphioxus, and the absence of a larval enterocœle and segmented mesoderm in Ascidians may perhaps do so too. But there remains one notorious obstacle to attempts to bring the two forms more closely into line: the grotesque asymmetry of the Amphioxus larva. Half a dozen theories have therefore been devised to explain it away as a larval adaptation of secondary import; but none has been proposed with much conviction or generally agreed upon. The interpretation to be set out here, so far from alienating Amphioxus from the ascidian, argues for an even closer affinity between them than is generally admitted.