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We will now compare the embryonic development of the two animals, and find to our great astonishment that the same embryonic form develops from the ovum of the Amphioxus as from that of the Ascidia a typical chordula.
In the middle line of its ventral side we find the hypobranchial groove. The ejection-opening of this peribranchial cavity corresponds to the branchial pore of the Amphioxus. In the adult Ascidia the branchial gut and the heart on its ventral side are almost the only organs that recall the original affinity with the vertebrates.
In other respects they exactly imitate the teasel cups showing thereby how these cups may probably have originated. In numerous cases of anomalies some accidental structures are parallel to specific characters, while others are not, being obviously injurious to their bearers. So it is also with the double ascidia.
II, § 1 and § 2 we have already referred to the value which Darwin, and more especially Häckel, lays on the relationship of the larva of the ascidia to the lancelet fish. Now the important testimony of K. E. von Baer, in his "Mémoires de l'Académie de St.
Ascidia of the snake-plantain or Plantago lanceolata are narrow tubes, because the leaves are oblong or lanceolate, while those of the broad leaved species of arrowhead, as for instance, the Sagittaria japonica, are of a conical shape. From the evidence of the lime-tree we may conclude that normal peltate leaves may have originated in the same way.
Now it is easily understood that the contact or the separation of the lobes must play a part in the construction of the ascidia, as soon as the margins grow together. Leaves which touch one-another, may be affected by the connation without any further malformation.
The further development of the Ascidia in detail has no particular interest for us, and we will not go into it. The chief result that we obtain from its embryology is the complete agreement with that of the Amphioxus in the earliest and most important embryonic stages.
Not less important are the later embryonic forms of the coelomula, the chordula, etc. Thus the embryology of the Amphioxus and the Ascidia has so much increased our knowledge of man's stem-history that, although our empirical information is still very incomplete, there is now no defect of any great consequence in it.
The other corresponds to the subintestinal vein and the ventral vessel of the worms. With the formation of these organs the progressive development of the Ascidia comes to an end, and degeneration sets in.
Those of the Croton, according to our knowledge regarding similar cases, must have arisen at once, and hence the conclusion that the ascidia of Nepenthes are also originally due to a sudden mutation. Interrupted leaves, with an ascidium on a naked prolongation of the midvein, are by no means limited to the Croton varieties.
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