. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates. 6 COMPARATIVE ANATOMY a central space, the primitive digestive cavity (arckenterpn). The opening of the latter to the exterior, where the two germinal layers are continuous, represents the primitive mouth or Nastopore, which is represented to a certain extent by the primitive streak of higher forms. From the ectoderm arise the epiderm and its derivatives, the entire nervous system, the sensory cells, the lens and certain muscles of the eye, the oral and anal involutions (stomodceum and procto- dceuiii), and the oral porti
. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates. 6 COMPARATIVE ANATOMY a central space, the primitive digestive cavity (arckenterpn). The opening of the latter to the exterior, where the two germinal layers are continuous, represents the primitive mouth or Nastopore, which is represented to a certain extent by the primitive streak of higher forms. From the ectoderm arise the epiderm and its derivatives, the entire nervous system, the sensory cells, the lens and certain muscles of the eye, the oral and anal involutions (stomodceum and procto- dceuiii), and the oral portion of the pituitary body attached to the brain. In an early stage the endoderm gives rise to an axial rod, the notochord (Figs. 6 and 7), and eventually to the epithelium of the Z'At-. I o FIG. 5.—GASTBULA. Blp, blastopore ; Ekt, ectoderm ; Ent, endoderm ; U, archenteron. greater part of the alimentary canal (Figs. 6 and 10) and its glands, including the thyroid, thymus, liver and pancreas, as well as to the epithelial parts of the gill-sacs and lungs. Though we may look upon the ectoderm and endoderm—that is, both the primary germinal layers—as arising primitively in the manner above described, various modifications occur, depending largely on the type of segmentation, and known as overgrowth (epiboly\ dclamination, and partial delamination. The middle layer or mesoderm is a secondary formation, and is phylogenetically younger than the other two germinal layers; both as regards the origin of its cells and histologically, it is of a compound nature, and thus forms a marked contrast to the two germinal layers proper. One of its first and most important functions is the formation of blood-cells; later it gives rise to the heart, vessels, and to nearly all the supporting and connecting substances (connective tissue, adipose tissue, cartilage, bone), serous membranes (peritoneum,. Please note that these images are extracted from scanned page images that may have been digitall
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