[211] The most important other instances in addition to that of the Cœlenterata which can be adduced in favour of the epiblastic origin of the mesoblast are the Bird and Mammal, in which according to the recent observations of Hensen for the Mammal, and Kölliker for the Mammal and Bird, the mesoblast is split off from the epiblast. If the views I have elsewhere put forward about the meaning of the primitive groove be accepted, the derivation of the mesoblast from the epiblast in these instances would be apparent rather than real, and have no deep morphological significance for the present question.
Other instances may be brought forward from various groups, but none of these are sufficiently well confirmed to be of any value in the determination of the present question.
[212] Vide Anthropogenie, p. 197.
[213] Vide Self, “Development of Elasmobranch Fishes,” Journal of Anat. and Phys. Vol. X. note on p. 682, and also Review of Professor Kölliker's “Entwicklungsgeschichte des Menschen u. d. höheren Thiere,” Journal of Anat. And Phys. Vol. X.
[214] Professor Haeckel speaks of the splitting of the mesoblast in Vertebrates into a somatic and splanchnic layer as a secondary process (Gastrula u. Eifurchung d. Thiere), but does not make it clear whether he regards this secondary splitting as taking place along the old lines. It appears to me to be fairly certain that even if the original unsplit condition of the mesoblast is to be regarded as a secondary condition, yet that the splitting of this must take place along the old lines, otherwise a change in the position of the body-cavity in the adult would have to be supposed—an unlikely change producing unnecessary complication. The succeeding argument is based on the assumption that the unsplit condition is a secondary condition, but that the split which eventually appears in this occurs along the old lines, separating the primitive splanchnopleure from the primitive somatopleure.
[215] Quart. Jl. of Micros. Science, July, 1875. [This Edition, No. VI.]
[216] Jenaische Zeitschrift, Vol. IX.
[217] Quart. Jl. of Micros. Science, Vol. XXV. 1874, and Phil. Trans. 1875.
[218] Archives de Zoologie, Vol. IV.
[219] Archiv f. Micr. Anat. Vol. XIII.