It has a wide coelomic aperture, and its walls for part of its course may become richly glandular, secreting albumen and sometimes an egg-shell, whilst its lower section may be dilated and as uterus1 retain the ovum while it undergoes development. Remnants of the Miillerian duct may be present in the male (hydatid of Morgagni: uterus masculinus, p. 30, ante, of Mammalia), but it is only in one Toad, Alytes, that it is connected directly to the testis and acts as vas deferens. It may so act in Dipnoi and some Ganoidei, but in this case it is not connected to the testis. In the male Gymnophiona and Bufo it persists and has a glandular function. Exceptions to the typical condition are (1) Cyclostomi and the Eels (Muraenidae), where sperm and ova are conducted outwards by an abdominal pore or pores (infra) as are the ova in a few Teleostei, e.g. Salmon: (2) Lepidosteus among Ganoidei and the Teleostei, where there are genital ducts continuous with the glands (see p. 89). Copulatory organs are formed as processes of the pelvic fins in Elasmobranchii and Holocephali; as organs varying in character attached to the cloaca in Amniota. In male Mammalia, except Prototheria, the organ is traversed by a continuous genito-urinary canal; similar but rudimentary organs are found in the female, and in some female Mammalia the clitoris (homologue of the penis) is traversed by the urinary canal (p. 36, ante). Accessory glands or diverticula are formed in connection with the male genito-urinary duct in most Mammalia, some of which have corresponding structures in the female.

1 This name is also generally given to the lower section of the oviduct in Aves, etc.

All the viscera except the nervous centres are contained in a body cavity or coelome inclosed between the body wall and their outer surfaces. It is lined by an epithelium and a delicate layer of connective tissue known as peritoneum. The viscera are kept in position and are sometimes freely suspended by dorsal folds of this peritoneum, known by special names according to the organ they support, - mesogastrium (stomach), mesentery (intestine), mesorchium (testis), mesovarium (ovary); besides several ligaments, e. g. those of the liver. Properly speaking the coelome is a paired cavity (p. 334), but traces of its paired character, due to the presence of a ventral in addition to a dorsal mesentery, are rarely found, e.g. in Dipnoi. A section of the coelome is inclosed to form the pericardium, and in Mammalia a thoracic region, including heart and lungs, is separated from an abdominal region containing the remaining viscera, by a fibro-muscular diaphragm. The coelome usually communicates with the lymphatic system.

And it may open externally in one of three ways: by posteriorly placed abdominal pores in Pisces, with the exceptions of some Elasmo-branchii and the vast majority of Teleostei, or by peritoneal canals in some Chelonia and the Alligator: through the nephrostomata when present: and through the Miillerian ducts of the female.

Each ovum in Vertebrata is surrounded during growth in the ovary by one or more layers of abortive ova, the cells of the tunica granulosa, the whole being known as the Graafian follicle. It is impregnated externally to the body in Cyclostomi, Ganoidei, Teleostei and Dipnoi, and in Anura among Amphibia; internally in all other Vertebrata. There are three ovular membranes known, (1) an external or vitelline persistent in Aves, (2) a middle striated membrane or zona radiata persistent in Teleostei and Mammalia and probably Amphibia, and (3) a delicate internal membrane present in Reptilia and Mammalia (? in Teleostei). But when ripe the ovum may be devoid of membranes as in Elasmobranchii. A single micro-pyle is present in Petromyzon, and many Teleostei, several in Acipenser. An adventitious coat of albumen is secreted round the ovum in Elasmobranchii, Holocephali, Dipnoi (Protopterus), Amphibia, Sauropsida and some Mammalia, to which may be added an egg-shell as in Elasmobranchii, Holocephali, and Sauropsida. The ovum contains much food yolk in Elasmobranchii, Holocephali, and Sauropsida; also in Teleostei. It is large in the three first named groups, small in the last named, but segmentation is partial in them all.

The ovum is smaller in other groups, and segmentation is either very unequal or, as in Mammalia, nearly equal. The gastrula is much modified. In Petromyzon, Acipenser and Amphibia there is a distinct invagination; one less distinct in Elasmobranchii; while a trace only of it is preserved in Sauropsida and Mammalia in the primitive streak which lies at the posterior end of the medullary groove.

There is an external yolk sac opening into the intestine with which its walls are continuous in Elasmobranchii, Lepidosteus, Teleostei, and Saurop-sida. It is known as umbilical vesicle in Mammalia. The part of it within the coelome frequently persists for a considerable period after birth or permanently as the omphalo-mesenteric duct (ductus vitello-intestinalis), especially in water-birds. The yolk is contained within the alimentary canal itself in other types, so far as is known. The embryo of Sauropsida and Mammalia is provided with two foetal envelopes, the amnion and the allantois. The former is produced by the upgrowth of a fold of the body wall (somatopleure). The fold surrounds the embryo on all sides and gradually incloses it. Fusion takes place on the dorsal aspect, and the inner limbs of the fold thus form a sac containing the embryo, and filled by a liquor amnii. The outer limbs are dissociated at the same time from the inner, and with the part of the somatopleure beyond the point where the fold first originated, form a second or outer sac, the false amnion or sub-zonal membrane. The allantois is a ventral diverticulum of the posterior end of the mesenteron.

It grows out into the space between the true and false amnion, and eventually comes into contact, except in a few instances, with either the whole or a part of the inner surface of the false amnion. The allantois is essentially a respiratory structure and is supplied with blood by two vessels, the umbilical arteries, derived from the iliac arteries. Its blood is returned at first by two umbilical veins homologues of the epigastric veins (supra, p. 353). One of them atrophies subsequently. In most Mammalia the 'chorion' formed by the union of the false amnion with the allantois comes into special relations with the uterine blood-vessels forming a 'placenta'. In some Mammalia the yolk sac is also in contact and fuses with a portion of the false amnion.