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As Gegenbaur very properly observes, "this gradual imbedding in the inner part of the body is a process acquired with the progressive differentiation and the higher potentiality that this secures; by this process the organ of greater value to the organism is buried within the frame." Central marrow of the human embryo from the seventh week, 4/5 inch long.
This simple primary pericardial cavity has been well called by Gegenbaur the "head-coeloma," and by Hertwig the "pericardial breast-cavity." As it now encloses the heart, it may also be called cardiocoel. The cardiocoel, or head-coelom, is often disproportionately large in the Amniotes, the simple cardiac tube growing considerably and lying in several folds.
Among the most prominent advocates of this view, we may name the late Sir Charles Lyell, Mivart, and Richard Owen, in England; and in Germany, Alexander Braun, Ecker, Gegenbaur, Oswald Heer, W. His, Nägeli, Rütimeyer, Schaaffhausen, Virchow, Karl Vogt, A. W. Volkmann, Weismann, Zittel, and here also Moriz Wagner, and among the philosophers, Eduard von Hartmann.
Gegenbaur, C., on the number of digits in the Ichthyopterygia; on the hermaphroditism of the remote progenitors of the vertebrata; two types of nipple in mammals. Gelasimus, proportions of the sexes in a species of; use of the enlarged chelae of the male; pugnacity of males of; rational actions of a; difference of colour in the sexes of a species of. Gemmules, dormant in one sex.
The agreement in the structure of the branchial gut of the Enteropneusts, Tunicates, and Vertebrates was first recognised by Gegenbaur ; it is the more significant as at first we find only a couple of gill-clefts in the young animals of all three groups; the number gradually increases.
Naturally, I do not mean that the Vertebrates have descended from the Tunicates, but that the two groups have sprung from a common root. The chordaea-theory received the most valuable and competent support from Carl Gegenbaur.
The thorough investigations of Gegenbaur have shown that the fish's fins, of which very erroneous opinions were formerly held, are many-toed feet. The various cartilaginous or bony radii that are found in large numbers in each fin correspond to the fingers or toes of the higher Vertebrates. The several joints of each fin-radius correspond to the various parts of the toe.
The thyroid appears in fishes, and Gegenbaur believes that it may have been a useful organ to the Tunicata in their former state of existence. Dr. Clevenger, in the American Naturalist for January, 1884, points out another curious structure in man, whose significance does not seem to have been previously observed. This is a strange and striking fact relating to the formation of the veins.
Yet these animals are far from being the root-forms from which all the Vertebrata have diverged, as is evidenced from the degree of specialization which their structure presents. If they have descended from such primitive forms as Professor Gegenbaur imagines, then they have built up a secondary serial homology a repetition of similar modifications fully as remarkable as if it were primary.
Gegenbaur referred on many other occasions to the close blood-relationship of the Tunicates and Vertebrates, and luminously explained the reasons that justify us in framing the hypothesis of the descent of the two stems from a common ancestor, an unsegmented worm-like animal with an axial chorda between the dorsal nerve-tube and the ventral gut-tube.
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