Fig. 387. Diagram of the primitive condition of the kidney in an Elasmobranch embryo.
pd. segmental duct. It opens at o into the body cavity and at its other extremity into the cloaca; x. line along which the division appears which separates the segmental duct into the Wolffian duct above and the Müllerian duct below; s.t. segmental tubes. They open at one end into the body cavity, and at the other into the segmental duct.
The next important change concerns the segmental duct, which becomes longitudinally split into two complete ducts in the female, and one complete duct and parts of a second duct in the male. The manner in which this takes place is diagrammatically represented in [fig. 387] by the clear line x, and in transverse section in [figs. 388] and [389]. The resulting ducts are (1) the Wolffian duct or mesonephric duct (wd), dorsally, which remains continuous with the excretory tubules of the mesonephros, and ventrally (2) the oviduct or Müllerian duct in the female, and the rudiments of this duct in the male. In the female the formation of these ducts takes place ([fig. 389]) by a nearly solid rod of cells being gradually split off from the ventral side of all but the foremost part of the original segmental duct. This nearly solid cord is the Müllerian duct (od). A very small portion of the lumen of the original segmental duct is perhaps continued into it, but in any case it very soon acquires a wide lumen ([fig. 389] A). The anterior part of the segmental duct is not divided, but remains continuous with the Müllerian duct, of which its anterior pore forms the permanent peritoneal opening[253] ([fig. 387]). The remainder of the segmental duct (after the loss of its anterior section, and the part split off from its ventral side) forms the Wolffian duct. The process of formation of these ducts in the male differs from that in the female chiefly in the fact of the anterior undivided part of the segmental duct, which forms the front end of the Müllerian duct, being shorter, and in the column of cells with which it is continuous being from the first incomplete.
Fig. 388. Diagrammatic representation of a transverse section of a Scyllium embryo illustrating the formation of the Wolffian and Müllerian ducts by the longitudinal splitting of the segmental duct.
mc. medullary canal; mp. muscle-plate; ch. notochord; ao. aorta; cav. cardinal vein; st. segmental tube. On the left side the section passes through the opening of a segmental tube into the body cavity. On the right this opening is represented by dotted lines, and the opening of the segmental tube into the Wolffian duct has been cut through; w.d. Wolffian duct; m.d. Müllerian duct. The section is taken through the point where the segmental duct and Wolffian duct have just become separate; gr. the germinal ridge with the thickened germinal epithelium; l. liver; i. intestine with spiral valve.
Fig. 389. Four sections through the anterior part of the segmental duct of a female embryo of Scyllium canicula.
The figure shews how the segmental duct becomes split into the Wolffian or mesonephric duct above, and Müllerian duct or oviduct below.
wd. Wolffian or mesonephric duct; od. Müllerian duct or oviduct; sd. segmental duct.
The segmental tubes of the mesonephros undergo further important changes. The vesicle at the termination of each peritoneal funnel sends a bud forwards towards the preceding tubulus, which joins the fourth section of it close to the opening into the Wolffian duct ([fig. 390], px). The remainder of the vesicle becomes converted into a Malpighian body (mg). By the first of these changes a tube is established connecting each pair of segments of the mesonephros, and though this tube is in part aborted (or only represented by a fibrous band) in the anterior part of the excretory organs in the adult, and most probably in the hinder part, yet it seems almost certain that the secondary and tertiary Malpighian bodies of the majority of segments are developed from its persisting blind end. Each of these secondary and tertiary Malpighian bodies is connected with a convoluted tubulus ([fig. 391], a.mg), which is also developed from the tube connecting each pair of segmental tubes, and therefore falls into the primary tubulus close to its junction with the segmental duct. Owing to the formation of the accessory tubuli the segments of the mesonephros acquire a compound character.