. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. 346 COMPARATIVE ANATOMY ST SSDS SG. a "later generation" of pronephric tubules. It appears more probable, however, that this organ originates independently from a part of the mesoblastic somites situated more dorsallythan that which gives rise to the pronephric tubules. Prmiitively, the mesonephros is strictly metameric, owing to the fact that each ot its tubules corresponds to the primary channel connecting the cavity of a somite with the unsegmented ccelome (Eig. 274). ihe loss of connection between these t
. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. 346 COMPARATIVE ANATOMY ST SSDS SG. a "later generation" of pronephric tubules. It appears more probable, however, that this organ originates independently from a part of the mesoblastic somites situated more dorsallythan that which gives rise to the pronephric tubules. Prmiitively, the mesonephros is strictly metameric, owing to the fact that each ot its tubules corresponds to the primary channel connecting the cavity of a somite with the unsegmented ccelome (Eig. 274). ihe loss of connection between these two sections of the primary coelome results in a series of segmental nephridia, each ot which opens into the body-cavity by a nephros- tome, while at its other, or blind end, it comes into connection with the prone- phric duct—or mesonephric duct as it must now be called (Fig. 275). The o-lomus of the pronephros is continued backwards, and in the region of the mesonephros breaks up into portions, or glomeruli, each of which is situated in a small cavity constricted off from the ca?.lome and opening into a mesonephric tubule, forming what is known as a Malpighian capsule (Figs. 274, 27-5). Each mesonephric tubule, then, in its primitive form, is made up of the following portions (Fig. 275):—(1) a funnel-shaped ciliated aperture, commu- nicating with the body-cavity (nepJiro- stome, or peritoneal funnel); (2) a rounded mass of capillaries {glomerulus), which is situated within a cavity {Mal- fighian capsule) derived from the ccfilome; and (3) a coiled glandular tuhule, opening into a collecting (me- sonephric) duct. Thus the mesonephros, as well as the pronephros, besides its main function of excreting waste pro- ducts by means of the epithelial cells lining the tubules, serves also to conduct water derived from the blood in the glomeruli, and peritoneal fluid, from the body. The mesonephros is of greatest importance in the Anamnia : in many Fishes it serves exclusively as a u
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