Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/481

Rh UBOGENITAL SYSTEM.] REPTILES 463 cloaca is somewhat oblique, and is situated in a somewhat funnel- shaped depression beside the opening of the ureter. In the Lacertilia the testes are spheroidal and almost symmetri- cally placed. The vasa efferentia come forth from their inner side and pass into a canal lying in a fold of peritoneum. This canal, the vas deferens, begins caecally much in front of the union of the vasa efforentia with it, and this anterior portion runs along the ventral side of the kidney in a fold of peritoneum and lias the appearance of a knot of tubuli or of a body transversely convoluted. The vas deferens passes backwards in numerous close-set convolu- tions, and often dilates towards its hinder end. It ultimately narrows, and opens on the cloaca, beside the ureter, on a small papilla. The Clielonia have testes which lie somewhat external to the kidneys as well as behind them. The vas defereiis terminates upon a small papilla in the cloaca, but its proximal end anterior to the junction with it of the vasa efferentia is wide, and consists of a very complicated mass of tubes, and it may have (as in Chelodina) five or six short caecal diverticula. The testes of the Crocodilia are of an elongated oval form, and lie partly in front of and partly on the inner side of the kidneys. The testis may be divided into two portions connected together dorsally. The vasa efferentia pass from the outer border of the testis to a vas deferens, which has rather thick walls with a marked dilatation towards its hinder end. It lies above the peritoneum, and opens at its distal end into a groove at the base of the penis. The Ovaries ami Oviducts. The ovaries are, as in the class Mammals, the glands which correspond with the testes, but which, unlike the latter, are not composed of a mass of tubules, while each does not discharge its product into a tube directly continuous with it (as does the testis into the vas deferens) but into a tube, the oviduct, the distal end of which is open and discontinuous with the ovary save for a broad fold of peritoneum which connects them. The ovaries always lie in the dorsal part of the abdominal cavity enclosed in peritoneum. In the Ophidia the ovaries are, like the testes, elongated and placed one in advance of the other. The right ovary is more voluminous, and the right oviduct is, of course, the longer one. The ova are so arranged within them as to form a longitudinal series. The proximal end of each oviduct shows a. transversely expanded wide opening. The oviduct passes backwards in con- volutions which are often spirally arranged, and it is very extensible, being lined with numerous close-set effaceable folds. The oviducts open into the cloaca by a crescentic fissure behind the opening of the rectum. In the Lacertilia, except the Amphisbrenians and probably some other Serpentiform Lizards, the ovaries have not their contained ova serially arranged. They may or may not be symmetrically disposed. The oviducts are usual!}' broad and spirally disposed. Unstriated muscular fibres often exist in the folds of membrane which support their convolutions. The Chelonia have ovaries symmetrically placed, often of a broad and flattened shape. The oviduct varies much in capacity in different parts of its course. The ovaries of the Crocodilia are com- pact in structure and somewhat more advanced in position than in most Reptiles. The oviduct is more uniform in diameter than in the Chdonia. The External Generative Organs. As has been already mentioned, all Reptiles have such external organs with the exception of Hatteria. In the rest of the class we always find such organs connected with the cloaca, and capable of being everted for use or retracted and hidden within or behind that chamber. The whole class is thus divisible into two groups according to the bifold or azygous condition of these parts. In the Lacertilice and Ophidia they are bifold, and consist in the males of two hollow, inver- tible, imperforate, cutaneous cones placed one on each side of the cloaca, containing erectile tissue and capable of being pro- truded or retracted by appro- priate muscles. The lining of each cone is continuous both with the external skin of the , . ., FIG. 29. Male copulatory organs of Lacer- adjacent parts and also with *?*< (after Lcydijr) P,Tp 2, organs of the mucous lining of the clonca. right and left sides between them is the A tortuous groove which begins anal a P ei 't"J'e ; pp, preanal plate, within the cloaca, at the aperture of the vas deferens, is continued on to the apex of the cone. When erected for use the lining membrane of the cones, with its grooves, becomes the external coat of each copulatory organ. In the females there are rudimentary r but quite similar organs, just as in Mammals the clitoris of the female is present as the rudimentary representative of the penis of the male. In the Ophidia these hollow penes may or may not bifurcate distally, and if they bifurcate then the groove bifurcates also. The lining membrane of the cone may be smooth, as in Python, or spiny, as in Trigonocephalus and Crotalus, or may have transverse rows of soft lamellae, as in Boa marina. The Lacertilia, like the Ophidia, may have each penis single to its apex or bifur- cating distally with various other minor modifications as to relative size, the form of the apex, and the appendages borne upon it. The cones are exceptionally short in the Chamseleons. The male Chelonia and Crocodilia have an azygous penis, and a corresponding rudimentary structure exists in the females. This organ lies at the ventral wall of the cloaca ; it is not hollow or evertible, but purely distensible and so erectile. It is therefore its external coat which is continuous with the lining of the cloaca. Muscles proceeding from beneath the hinder trunk vertebra are inserted into it. The penis is imperforate, but two ridges are continued on to it from the wall of the cloaca, and so form a groove, and during erection a temporary canal, which passes along it from root to apex. At its distal end there is a prominence which, though imperforate, may remind us of the "glans" of the Mam- malian penis. The whole body of the organ, including this distal prominence, contains erectile tissue. In the Chelonia the penis may divide distally, and if so the groove it bears divides also. In Chelodina it has an undivided distal end and two lateral pro- cesses. In Trionyx it subdivides distally into four terminal parts. The penis of the Crocodile has a deep groove which reaches to the extreme end of the penis, a quasi-glans projecting freely beneath it. A few Reptiles have secondary sexual characters notably the Chamseleons, the males of which alone have the horns and other cephalic appendages 'which characterize certain species. In most Reptiles, however, the two sexes have hardly any distinctive external characters. Embryology. Most of the Reptilia are oviparous, but certain of the Lacertilians and many Ophidians, notably Vipers and Sea-Snakes, hatch their eggs before they are laid ; that is, they are ovoviviparous. The oviduct supplies the ovum during its exit with an albuminous investment, the white of the egg, and with a shell, or testa, which may be thin and flexible, as in the Lacertilia and Ophidia, or hard and calcareous, as in the Crocodilia and Chelonia, the eggs of which animals much resemble those of Birds. . The embryo Reptile closely resembles in its general features the embryo Bird, and, as the process of develop- ment of the chick (which can be so conveniently studied) is now comparatively thoroughly well known (see REPRO- DUCTION), we cannot here make more than a cursory mention of the leading structural changes. Inasmuch as embryonic Reptiles thus resemble embryonic Birds, it of course follows that they present all those features which embryonic Birds share with Vertebrate animals generally, such as the segmentation of the yolk, the resulting forma- tion on its surface of the blastoderm, and its division into the three germinal layers. As in Birds, the yolk segmen- tation is meroblastic. The blastoderm spreads rapidly over the yolk, but before it has half enclosed it a pyriform patch, the embryonic shield, or area pellucida, appears at its centre. The head of the embyro is formed at the broader end of the pyriform patch. Towards its hinder end a streak of the epiblast is formed, the primitive groove, 1 and then in front of it another longitudinal indentation, the " medullary groove," the walls of which are the " medullary plates " ; and it is this groove and plates with the parts immediately sub- jacent which lay the foundation of the developing body. The lining of the medullary groove becomes the cerebro- spinal axis, while a longitudinal cellular rod which is formed beneath it, and which is called the notochord, lays the foundation of the future axial skeleton as far forward as the hinder margin of the cranial support of the pituitary body of the brain. The anterior end of the medullary 1 Supposed to be an indication of a form of iuvagination of the germinal layers characteristic of lower forms.