The renal organs of vertebrates present many other features of embryological interest. In various of the more archaic types the rudiments of the first pronephric tubules, in the form of little outgrowths of the somatic endoderm, bend backwards at their outer ends and become joined together, forming in this way the rudiment of a longitudinal duct (archinephric duct) which gradu ally extends back, receiving the successive tubules which undergo fusion with it, and eventually opens into the cloaca. This opening of the tubules into a longitudinal duct instead of directly to the exterior constitutes one of the striking differences between the Vertebrata and the Annelida.
In elasmobranch fishes the archinephric duct becomes split longitudinally into two—a Miillerian duct, into which opens the persistent pronephric nephrostome, and a Wolffian duct, into which open the tubules of the opisthonephros. Functionally, the former becomes the oviduct. In the Amniota the functional tubules of the metanephros open into a third duct—the ureter— while the Wolffian duct now functions exclusively as a vas deferens or male genital duct.
As regards the evolutionary history of these ducts, the known facts of embryology support the view that (I) the archinephric duct came into existence through the external opening of each tubule becoming shifted back so as to open into its successor, (2) the Miillerian and Wolffian ducts became separated by a process of splitting, and (3) the ureter arose from the collecting tube or trunk portion of the large tree-like mass of tubules con stituting the metanephros and formed by the enlargement and branching of the last tubule of the opi§thonephros.
Finally a noteworthy feature of the renal organs of vertebrates is that the portion of coelome in proximity to the nephrostome tends to become isolated as a small spheri(al chamber, the Mal pighian body : a small portion of the lining of this, in which the power of secreting watery coelomic fluid has become specially concentrated, bulges into the cavity of the Malpighian body as the spherical glomerulus containing an arterial network supplied from the dorsal aorta. The separation of Malpighian body from the main splanchnocoele becomes more and more pronounced as the evolution of the renal organs proceeds.
The Gonad—ovary or testis—is, as in other groups, a develop ment of the coelomic epithelium. Situated just ventral to the segmented portion of the mesoderm, it shows in a few cases dis tinct traces of segmentation in early stages of its development.
In the female, the reproductive cells (eggs or macrogametes) are still shed into the splanchnocoele, finding an exit through the Miillerian ducts. In the male, however, the fertile portion of coelomic epithelium (testis) becomes shut off as an isolated chamber into the cavity of which the reproductive cells (sperma tozoa or microgametes) are shed. They eventually reach the vas deferens by way of fine tubular channels (vasa efferentia), which arise in the embryo as outgrowths from the wall of certain of the Malpighian bodies.
The mesenchyme cells, distributing themselves through the body of the embryo, settle down into spongy connective tissue which forms a support and backing to the various developments of ectoderm and endoderm. As development proceeds, special tracts take on special characters—fatty tissue, tendon, ligament and so on—but there are two developments of the mesenchyme which are of special importance. One of these is characterized by its strands becoming hollow vessels in which fluid circulates, the other by its strands becoming rigid and constituting a skeletal or supporting framework to the body.