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transient apparatus exhibits signs of degeneration, i. e. of being in a somewhat vestigeal form. And in another section it is briefly shown, that there exists no morphological resemblance whatever between the transient and permanent nervous systems, and that the one cannot be regarded as an earlier developed portion of the apparatus, which succeeds it.
The concluding section of the memoir treats, in a preliminary fashion, of development by substitution of organisms, or antithetic alternation of generations.
Among other things an attempt is made to show how in the development of the skate a sexual form (the skate embryo) arises by substitution on an asexual foundation or larva, the former gradually replacing the latter.
There would appear to be, in disguised form, two generations contained in what is usually designated “a young skate embryo". The one generation (here the sexual one) begins to arise on the other (the asexual one) at a very early period after the fertilisation of the egg. At first the asexual form, or reduced larva, obtains the start, and gets into a position to develop its nervous system. Meanwhile the future sexual form has been making slow but sure progress in its development, and, anon, a period arrives when the two cannot co-exist as of equal functional value. As shown by commencing degeneration of its nervous system, the asexual form, or larva, begins to get suppressed, and in the skate-development this appears to be brought to pass by a sudden spurt on the part of the sexual generation.
Previously the organism apparent to our vision lacked many of the most essential characters of the skate, and its sex could not be determined. Now, and now for the first time, it becomes markedly flattened, its pectoral fins commence their forward growth, its organs are getting into something like the adult form and histological condition, the cells of the cord and of the spinal ganglia are becoming distinctly ganglionic, the permanent canal of the central nervous system is in course of formation, and, of some import, the sex of the organism becomes obvious even from the external characters.
These points indicate the great changes in operation, changes all tending towards the attainment of the adult form. The thesis is stated that all Metazoan development from the egg appears to be fundamentally an antithetic alternation of generations, and that directly or indirectly there is invariably a substitution of organisms.
It is also suggested that, while in the higher plants the sexual generation becomes more and more reduced with the higher evolution of the asexual generation, in the animal kingdom the conditions would appear to be reversed, as we ascend the scale; for here the sexual generation would be the one to attain higher and higher morphological differentiation with a progressive reduction and degeneration of the asexual generation or larva.
Table of sizes, number of somites and state of gill-clefts or pouches
Appendix b. The comparison with Rall's finds in Pristiurus in the following summary may be useful :
It is difficult to bring the two tables into exact correspondence, for there is some variation on Raja in the period of the actual rupture of the various pouches. Thus in No. 564 the spiracle has lagged behind and is still closed, although the third branchial is open on one side. RABL correct ly states that the spiracle usually only opens after the first and second branchials.
List of literature cited.
1. BALFOUR, F. M., A monograph of the development of Elasmobranch
Fishes, 1878. 2. BEARD, J., The early development of Lepidosteus osseus, in: P. R.
Soc. London, 1889, V. 46, p. 108–118. 3. The transient ganglion cells and their nerves in Raja batis,
in: Anat. Anz., (1892) V. 7, p. 191-206. 4. BURCKHARDT, K. R., Histologische Untersuchungen am Rückenmark
der Triton., in: Arch. mikr. Anat., V. 34, p. 131-156, with
2 plates, 1889. 5. Dohrn, Anton, Studien zur Urgeschichte des Wirbelthierkörpers.
No. 17. Nervenfaser und Ganglienzelle. Histologische Untersuchungen, in: Mt. Zool. Station Neapel, V. 10, p. 256—341, with
7 plates, 1891. 6. FRITSCH, G., Ueber den Angelapparat des Lophius piscatorius, in:
SB. Akad. Berlin, 1884, p. 1–7. - Also: Ueber einige bemerkenswerthe Elemente des Centralnervensystems von Lophius piscatorius
L., in: Arch. mikr. Anat., V. 27, 1886, p. 13–31, tab. 3 and 4. 7. KLEINENBERG, N., Die Entstehung des Annelids aus der Larve von
Lopadorhynchus, in: Z. w. Zool., V. 44 (1886), p. 1–227, with
16 plates. 8. KÖLLIKER, A. von, Handbuch der Gewebelehre des Menschen, 6. Aufl.
V. 2, (1893) 9. KUPFFER, CARL von, Die Entwicklung der Kopfnerven der Verte
braten (Referat), in: Verh. Anat. Ges., 1891, p. 22–55. 10. MAYER, Paul, Die unpaaren Flossen der Selachier, in: Mt. Stat.
Neapel, v. 6, (1885), p. 217—285, with 5 plates. 11. ROHDE, Emil, Histologische Untersuchungen über das Nerven
system von Amphioxus, in: Zool. Anz., V. 11 (1888), p. 190—196, and under the same title in extenso, in: SCHNEIDER’s Zool. Beitr.,
V. 2, 1888, p. 169-211, with 2 plates. 12. RETZIUS, Gustav, Die nervösen Elemente im Rückenmark der
Knochenfische, in: Biol. Untersuchungen, N. F. V. 5, 1893. 13. Rohon, VICTOR, Zur Histogenese des Rückenmarks der Forelle, in:
SB. Ak. Wien, 1884, p. 39–57, with 2 plates. 14. STIEDA, LUDWIG, Studien über den Amphioxus lanceolatus, in: Mém.
Acad. St. Pétersbourg, V. 19, 1873, with 4 plates. 15. STRASSER, H., Alte und neue Probleme der entwicklungsgeschicht
lichen Forschung auf dem Gebiete des Nervensystems, in: Ergeb. Anat., V. 1, 1892, p. 721-769.
Explanation of Plates.
List of reference letters. C. C capsule cells,
n. p nerve-process of a ganglionct cartilage,
cell, d. c degenerating cell,
s.d segmental duct, ep epiblast,
S. n sub-epiblastic transient g gut,
nerve, gl. c transient ganglion-cell, sp.c spinning ganglion-cell, gl.c.c centrally lying lying transient
in transient nerve, ganglion-cell,
v vacuole in a degenerating 1.7 lateral line,
ganglion-cell, l.n lateralis nerve,
w.gl. c wandering transient ganmp myotome,
glion-cell. n notochord,
Unless otherwise described, all the figures relate to Raja batis. All the figures were drawn with the aid of an ABBE camera. The following lenses were used, and their approximate magnifications are given below. Zess A, ocular 2 = 50 diameters; HARTNACK 5, oc. 2 140. 225; Zeiss D, compensat. oc.
4 230. ZEISS F,
500; ZEISS F, compensat. oc. 4 525. SEIBERT 4 mm apochromatic, oc. 2
225. LEITZ 2 mm pantachromatic, oc. 2 500.
The figures on Plates 1, 2, 3, 4 have been reduced by the lithographer to two-thirds of their former size. This is indicated in the description by such figures as 225 X . Except where otherwise stated, the figures are portions of transverse sections.
oc. 2 oc. 2
Fig. 1. Shows the dorsal portion of the spinal cord, where certain of the cells have taken on ganglionic characters. To the left of the figure are two such, which have spun out processes towards the muscle plate. The cells of the future spinal ganglion are still migrating from the lips of the cord. Embryo No. 155. About 9,5 mm, 100 somites. Magnif. 500 X .