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Updated: May 19, 2025
Between the ectoderm and entoderm of the gastrula, in the space occupied by the supporting membrane of hydra, a new layer of cells, the mesoderm, appears. This has been produced by the rapid growth and reproduction of certain cells of the entoderm which have migrated, so to speak, into this new position.
Simultaneously with the construction of the medullary tube we have in the Amphioxus-embryo the formation of the chorda, the coelom-pouches, and the mesoderm proceeding from their wall. These processes also take place with characteristic simplicity and clearness, so that they are very instructive to compare with the vermalia on the one hand and with the higher vertebrates on the other.
Of the two parts that compose the comb-shaped primitive kidney the longitudinal channel, or nephroduct, is always the first to appear; afterwards the transverse "canals," the excreting nephridia, are formed in the mesoderm; and after this again the Malpighian capsules with their arterial coils are associated with these as coelous outgrowths.
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