The Mesoderm and the Coelom of Vertebrates

Abstract

fHE morphology of the mesoderm is one of the most vexed questions of the day. Scarcely an embryologist can be found who has not published opinions on this question consider-ably at variance with those of most others. It has been main-tained that the mesoderm arises from the ectoderm; that it arises from the the entoderm, or from both; from neither, but from two special segmentation spheres; that it has a double origin, part coming from the blastoderm, part from the yolk; and even that -there is no mesoderm. We now know positively that in all vertebrates there is a dis-tinct and unmistakable mesoderm, which spreads out from the primitive streak in all directions, and has distinctive histological characteristics. Two large cavities appear in this mesoderm on either side of the median axial line. The mesodermic cells which bound these two cavities assume an epithelial arrangement, and are designated as the mcso/zeh/iunm; the cavities constitute the cwlo,; the remainder of the mesoderm is known as the mesen-cizyma, and corresponds to the embryonic connective tissue of older writers. The mesothelium at various points throws off cells, which are added to the mesenchyma. We have accordingly two distinct phases to study, viz., the origin of the mesoderm, and the differentiation of the mesenchyma and mesothelium, and together with the latter the formation of the ccelomatic cavities. This content downloaded from 065.112.008.075 on February 13, 2018 09:18:34 AM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-878 'The Amertican Naturalist. [October, I. ORIGIN OF THE MESODERM. Mesoderm of Elasmobranchs.-In the cartilaginous fishes the mesoderm arises from the entoderm close to the ectental line. The observations of Balfour in his monograph, 2 (see also his works, I., 246-268), established the fact that the mesoderm appears after the two primary layers and is connected with the entoderm. This fact has since been abundantly confirmed (see Kollmann, 25; Swaen, #i; Rickert, 32, 33; Rabl, 29; D.