DEFECTIVEPLUTEUS LARVÆFROM ISOLATED BLASTOMERES OFARBACIAANDECHINARACHNIUS

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Abstract
Results of Von Ubisch''s work on the normal development of sea urchin''s eggs showing that, contrary to Boveri, the 1st cleavage plane bears no relation to the median plane of the larva, necessitate re-study of the development of isolated blastomeres. The author studied the development of the first 2 blastomeres of Arbacia and Echinarachnius eggs separated mechanically. Of blastomeres which developed to ciliated larvae there were 3 classes with respect to the skeleton: those with no skeleton; those in which the skeleton was defective or partial; those with a complete skeleton of] size. When both blastomeres of the same egg developed, there were again 3 main groups: a few which developed 2 plutei with perfect skeletons; a much larger number which formed 2 plutei each with an incomplete skeleton; a group which formed 1 perfect pluteus and 1 larva without a skeleton. In several cases 2 larvae were found from such pairs with skeletons which were complementary. These findings indicate that at the 1st cleavage of these eggs there is already localized a layer of skeleton-forming material at right angles to the primary egg axis and in the vegetative half of the egg, which may be cut at any angle by the 1st cleavage plane and unequally distributed to the blastomeres. A bilateral organization of this material is suggested, although not often apparent. The eggs of these sea urchins are, therefore, not isotropic, in the sense of Driesch.

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