I. (1) The normal development of Astropecten irregularis is described up to the twenty-fifth day.

(2) About a third of the larvae possessed two pore-canals, and larval twinning was observed in two cases.

II. There is insufficient evidence for believing that normal Echinoderm larvae possess a ‘latent’ bilateral symmetry.

III. The following provisional conclusions are reached regarding duplicity in Echinoderm larvae:

(1) The various kinds of duplicity form a series.

(2) They are of the same nature as those found in vertebrate embryos, and are probably due to similar causes.

(3) They may be determined by

(a) Alteration of the polarity of the egg ;

(b) Interference with processes of early development affecting gastrulation ;

(c) Physiological inhibition or mechanical deformation of the tip of the archenteron.

(4) Their ultimate facies, in the case of (c), is determined largely by excess or defect of nutrition.

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