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Updated: May 15, 2025
Inside the membrane that surrounds the ovum, the stem-cell of the Ascidia, after fecundation, passes through just the same metamorphoses as the stem-cell of the Amphioxus. It undergoes total segmentation; it divides into two, four, eight, sixteen, thirty-two cells, and so on. By continued total cleavage the morula, or mulberry-shaped cluster of cells, is formed.
Man is only a more advanced stage of the vertebral type that we find unmistakably in the Amphioxus in its characteristic features. We need only recall the picture of the ideal Primitive Vertebrate given in a former chapter, and compare it with the lower stages of human embryonic development, to convince ourselves of our close relationship to the lancelet.
Transverse section of an Amphioxus-larva, with five gill-clefts, through the middle of the body. The small particles that the Amphioxus takes in with the water infusoria, diatoms, particles of decomposed plants and animals, etc. pass from the gill-crate into the digestive part of the canal, and are used up as food.
Goodsir long ago insisted upon the resemblance between Amphioxus and the Ascidians; but the notion of a genetic connection between the two, and especially the identification of the notochord of the Vertebrate with the axis of the caudal appendage of the larva of the Ascidian, is a novelty which, at first, takes one's breath away.
The branchial or head-gut of the Ascidia is small at first, and opens directly outwards only by a couple of lateral ducts or gill-clefts a permanent arrangement in the Copelata. The gill-clefts are developed in the same way as in the Amphioxus. As their number greatly increases we get a large gill-crate, pierced like lattice work.
It was a fortunate accident that Kowalevsky first examined just those larger specimens of the Ascidiae that show most clearly the relationship of the vertebrates to the invertebrates, and the larvae of which behave exactly like those of the Amphioxus in the first stages of development.
Especially in the lowest of all the vertebrates, the lancelet or Amphioxus, we have the oldest stem-forms completely preserved in the embryonic development. We also find important evidence in the fishes, which stand between the lower and higher vertebrates, and throw further light on the course of evolution in certain periods.
Here he adopts the investigations of A. Kowalewsky, and the deductions of Häckel founded upon them, concerning the larva of the ascidiæ, a genus of marine mollusca of the order tunicata, and sees in a cord, to be found in this larva, most decided relationship to the spine of the lancelet fish or amphioxus, the lowest of all the vertebrates, it being yet doubtful whether it belongs at all to the vertebrates.
Another favourite argument is the fact of “Pædogenesis.” Certain animals, such as Amphioxus lanceolatus, Peripatus, and certain Medusæ, are very frequently brought forward as examples of persistent primitive stages and “transitional connecting links.” But considered from the point of view of Pædogenesis, they all assume quite a different aspect, and seem rather to represent very highly evolved species, and to be, not primitive forms, but conservative and regressive forms.
Perhaps the most startling proposition of all those which Professor Haeckel puts before us is that which he bases upon Kowalewsky's researches into the development of Amphioxus and of the Ascidioida, that the origin of the Vertebrata is to be sought in an Ascidioid form.
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