restricted to a comparatively short anteroposterior extent. In Selachians the vasa efferentia are restricted to the anterior end of testis and kidney, and are connected by a longitudinal canal ending blindly in front and behind. The number of vasa efferentia varies and in the rays (Raia, Torpedo) may be reduced to a single one opening directly into the front end of the mesonephric duct. The anterior portion of the mesonephros is much reduced in size in correlation with the fact that it has lost its renal function. The hinder part, which is the functional kidney, is considerably enlarged. The primary tubules of this region of the kidney have undergone a modification of high morphological interest. Their distal portions have become much elongated, they are more or less fused, and their openings into the mesonephric duct have undergone backward migration until they open together either into the mesonephric duct at its posterior end or into the urinogenital sinus independently of the mesonephric duct. The mesonephric duct is now connected only with the anterior part of the kidney, and serves merely as a vas deferens or sperm duct. In correlation with this it is somewhat enlarged, especially in its posterior portion, to form a vesicula seminalis.
The morphological interest of these features lies in the fact that they represent a stage in evolution which carried a little farther would lead to a complete separation of the definitive kidney (metanephros) from the purely genital anterior section of the mesonephros (epididymis), as occurs so characteristically in the Amniota.
Dipneusti.—In Lepidosiren[1] a small number (about half a dozen) of vasa efferentia occur towards the hind end of the vesicular part of the testis and open into Malpighian bodies. In Protopterus the vasa efferentia are reduced to a single one on each side at the extreme hind end of the testis.
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| Graham Kerr, Proc. Zool. Soc. London. | ||||||||||||||||
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Fig. 12.—Diagram illustrating Connexion between Kidney and Testis in Various Groups of Fishes. | ||||||||||||||||
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Teleostomi.—In the actinopterygian Ganoids a well-developed testicular network is present; e.g. in Lepidosteus[2] numerous vasa efferentia arise from the testis along nearly its whole length and pass to a longitudinal canal lying on the surface of the kidney, from which in turn transverse canals lead to the Malpighian bodies. (In the case of Amia they open into the tubules or even directly into the mesonephric duct.) In the Teleosts and in Polypterus there is no obvious connexion between testis and kidney, the wall of the testis being continuous with that of its duct, much as is the case with the ovary and its duct in the female. In all probability this peculiar condition is to be explained[3] by the reduction of the testicular network to a single vas efferens (much as in Protopterus or as in Raia and various anurous Amphibians at the front end of the series) which has come to open directly into the mesonephric duct (cf. fig. 12).
Organs of the Mesenchyme.—In vertebrates as in all other Metazoa, except the very lowest, there are numerous cell elements which no longer form part of the regularly arranged epithelial layers, but which take part in the formation of the packing tissue of the body. Much of this forms the various kinds of connective tissue which fill up many of the spaces between the various epithelial layers; other and very important parts of the general mesenchyme become specialized in two definite directions and give rise to two special systems of organs. One of these is characterized by the fact that the intercellular substance or matrix assumes a more or less rigid character—it may be infiltrated with salts of lime—giving rise to the supporting tissues of the skeletal system. The other is characterized by the intercellular matrix becoming fluid, and by the cell elements losing their connexion with one another and forming the characteristic fluid tissue, the blood, which with its well-marked containing walls forms the blood vascular system.
Skeletal System.—The skeletal system may be considered under three headings—(1) the chordal skeleton, (2) the cartilaginous skeleton and (3) the osseous skeleton.
1. Chordal Skeleton.—The most ancient element of the skeleton appears to be the notochord—a cylindrical rod composed of highly vacuolated cells lying ventral to the central nervous system and dorsal to the gut. Except in Amphioxus—where the condition may probably be secondary, due to degenerative shortening of the central nervous system—the notochord extends from a point just behind the infundibulum of the brain (see below) to nearly the tip of the tail. In ontogeny the notochord is a derivative of the dorsal wall of the archenteron. The outer layer of cells, which are commonly less vacuolated and form a “chordal epithelium,” soon secretes a thin cuticle which ensheaths the notochord and is known as the primary sheath. Within this there is formed later a secondary sheath, like the primary, cuticular in nature. This secondary sheath attains a considerable thickness and plays an important part in strengthening the notochord. The notochord with its sheaths is in existing fishes essentially the skeleton of early life (embryonic or larval). In the adult it may, in the more primitive forms (Cyclostomata, Dipneusti), persist as an important part of the skeleton, but as a rule it merely forms the foundation on which the cartilaginous or bony vertebral column is laid down.
2. Cartilaginous or Chondral Skeleton.—(A) Vertebral column.[4] In the embryonic connective tissue or mesenchyme lying just outside the primary sheath of the notochord there are developed a dorsal and a ventral series of paired nodules of cartilage known as arcualia (fig. 13, d.a, v.a). The dorsal arcualia are commonly prolonged upwards by supradorsal cartilages which complete the neural arches and serve to protect the spinal cord. The ventral arcualia become, in the tail region only, also incorporated in complete arches—the haemal arches. In correlation with the flattening of the body of the fish from side to side the arches are commonly prolonged into elongated neural or haemal spines.
The relations of the arcualia to the segmentation of the body, as shown by myotomes and spinal nerves, is somewhat obscure. The mesenchyme in which they arise is segmental in origin (sclerotom), which suggests that they too may have been primitively segmental, but in existing fishes there are commonly two sets of arcualia to each body segment.
In gnathostomatous fishes the arcualia play a most important part in that cartilaginous tissue derived from them comes into special relationships with the notochord and gives rise to the vertebral column which functionally replaces this notochord in most of the fishes. This replacement occurs according to two different methods, giving rise to the different types of vertebral column known as chordacentrous and arcicentrous.
