A lymphatic tissue, known as adenoid or reticular tissue, consisting of a net-work of cells, which bud and form lymph or white corpuscles, is greatly developed in the submucous coat of the intestine, and as masses or lymphatic glands here and there in the course of the lacteal and lymphatic vessels of higher Vertebrata.
The supra-renal bodies usually found in close apposition with the kidneys or genitalia, appear to consist of a portion (the medulla in higher forms) derived from the sympathetic nervous ganglia: and another portion, the cortex, derived from mesoblast, and in higher forms in immediate connection with the vena cava inferior and cardinal veins. The two parts remain separate in Elasmobranchii - the mesoblastic as the inter-renal body, the nervous as a series of paired bodies connected with the intercostal branches of the aortal.
All Vertebrata possess a paired kidney. This organ with its duct has a complex history. The primitive kidney duct is known as the 'segmental duct;' it extends from the anterior region of the coelome to the primitive cloaca, opening into both. This duct in Ichthyopsida becomes divided except in Cyclostomi (? Teleostei and Ganoidei; see general account of Pisces) into two ducts, a Mullerian duct with a coelomic aperture, and a closed Wolffian duct. In the Amniota the two appear to develope partially, or in some cases perhaps wholly, independently of each other. With the fore part of the segmental duct is connected in the embryoes or larvae of Ichthyopsida except Elasmobranchii a pronephros. It has the form of 1-5 tubes produced from the extremity of the segmental duct, inclosed in a special section of the coelome, with a vascular ridge or glomerulus opposed to the apertures of the tubes. It appears to atrophy in all cases. There is a somewhat similar structure in the Chick, connected however with the Mullerian duct and devoid of a glomerulus.
A mesonephros, or Wolffian body, is developed in all Vertebrata posteriorly to the pronephros, but it is only an embryonic organ in the Amniota with the exception of its genital region in the male (infra). It consists of a series of tubules derived from the peritoneum or mesoblast opening into the segmental duct in Cyclostomi; into the Wolffian or mesonephric duct in other types (Teleostei and Ga-noidei ?). These tubules are arranged segmentally, one to each segment in the embryo Elasmobranch, and the adult Myxine, and Gymnophiona, but in other types they are, especially in the posterior segments of the body, more numerous than the segments. Secondary and tertiary tubules are added to those first formed. Each tubule is typically composed of (1) a ciliated funnel or nephrostome opening into the coelome; (2) a Malpighian body, i. e. a dilatation with a tuft of blood-vessels projecting into it: (3) a coiled glandular tube or tubulus uriniferus; and (4) a collecting tube. The nephrostomata persist in Elasmobranchii and Amphibia in connection with more or fewer of the primary tubules. Present in the embryo in many other instances, they are always aborted at a certain stage of growth.
The anterior part of the mesonephros becomes connected with the testis in the male of Elasmobranchii, certain Ganoidei (? all), all Amphibia and Amniota. The corresponding portion in the female is modified or nearly aborted in the Ichthyopsida, and persists in Amniota as a variable rudiment (ep-oophoron of Mammalia). The posterior non-sexual part of the mesonephros becomes partially or wholly independent of the fore part in Elasmobranchii and communicates with the Wolffian duct by one or more independent ureters, specialised collecting tubes. In Amphibia it remains continuous with the sexual region, but is strongly marked off from it in Urodela; and its ducts sometimes tend to unite inter se before they fall into the Wolffian duct. The permanent kidney of the Amniota is a meta-nephros. It appears to be 'a specially differentiated posterior section of the mesonephros' (Balfour) developing at a later period, comparable to the posterior region of the mesonephros of Elasmobranchii. The ureter in this case is an outgrowth of the Wolffian duct, the collecting tubes of the ureter; the gland tubes (tubuli uriniferi) and Malpighian bodies are formed independently (?) out of the mesoblast. There are no nephrostomata.
Remnants of the non-sexual part of the mesonephros may persist (par-epididymis, par-oophoron, of Mammalia).
1 Weldon believes that the supra-renal bodies represent the pronephros in Cyclostomi; the pro-plus part of the meso-nephros in Teleostei; a part of the mesonephros in secondary connection with the sympathetic ganglia in Elasmobranchii and higher Vertebrata. P. R. S. xxxvii. 1884; see also Q. J. M. xxiv. 1884.
The primitive connection of the segmental duct or of its derivatives, the Miillerian and Wolffian ducts, as well as of the ureter in Amniota with a cloaca common to both anus, urinary and genital ducts, is retained in Cyclostomi, Elasmobranchii, and Dipnoi among Pisces, in all Amphibia and Sauropsida and in the Prototheria among Mammalia. The cloaca is divided into an anal section and a urogenital section in all Ganoidei, many Teleostei and nearly all Mammalia. In Holocephali and a few Teleostei among Pisces, and a few female Mammalia the anal, genital and urinary apertures are independent. The anal aperture of Mammalia is always placed behind the genito-urinary apertures, in front in all the other classes. But in all Vertebrata the urinary aperture is anterior to the genital. The ureters in Mammalia, except Prototheria, open into a urinary bladder, a persistent part of the allantois (infra). The urinary bladder so called of Teleostei and Ganoidei is a dilatation or outgrowth of the urinary ducts themselves.
The sexes are distinct in Vertebrata. Hermaphroditism may occur as an abnormality, as a rule only in one Anuran and two Teleosteans. The testis and ovary are alike derived from a thickened ridge of coelomic epithelial cells. They are as a rule retained in the coelome, but in many Mammalia the testes shift their position either temporarily or permanently and are lodged in a pouch or caecum of the body wall which includes them and a portion of the coelome, and is known as scrotum. The sperm is shed into the coelome in Cyclostomi, in Dipnoi (?) and perhaps some Ganoidei: but as a rule it is transported to the exterior by a network of tubes derived from and in connection with the anterior region of the mesonephros and thence through the Wolffian duct. This region of the mesonephros persists in the Amniota and forms the coni vasculosi and vasa recta, while the Wolffian duct becomes epididymis and vas deferens. Remnants of the Wolffian duct are sometimes found in female Amniota (tube of ep-oophoron; duct of Gartner in Mammalia). The ova are typically shed into the coelome and are thence transported outwards by the Miillerian duct, which is known as Fallopian tube or oviduct.