358 THE ANIMAL KINGDOM. 



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 per- 

 manently 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 Mam- 

 malia the ' chorion ' formed by the union of the false amnion with the 

 allantois comes into special relations with the uterine blood-vessels form- 

 ing a ' placenta.' In some Mammalia the yolk sac is also in contact and 

 fuses with a portion of the false amnion. 



The majority of Vertebrata are oviparous. Mammalia except Proto- 

 theria are viviparous, and instances of viviparity occur among Lacertilia, 

 Ophidia, Urodele Amphibia, Teleostei and Elasmobranchii. 



There are two divisions of Vertebrata, ft&Amniota and ihe Anamniota 

 s. Icthyopsida. 



Lehrbuch der Vergleich. Anat. der Wirbelthiere, Wiedersheim, Jena, 1883. 

 Anatomy of vertebrated animals, Huxley, London, 1871. Anatomy of Vertebrates, 

 Owen, 3 vols. London, 1866-68. 



Morphology of skull, Parker and Bettany, London, 1877. 



Pineal gland, Ahlborn, Z. W. Z. xl. 1884 ; and eye, De Graaf, Z. A. ix. 1886; 

 Spencer, Nature, xxxiv. 1886. 



Cranial nerves, Milnes, Marshall, and Spencer, Q. J. M. xxi. 1881 ; cf. Beard, 

 'Branchial sense-organs,' &c., Q. J. M. xxvi. with lit. p. 148-151. 



Mucous membrane of nose in Pisces and Amphibia, Blaue, Arch. f. Anat. u. 

 Physiol., Anat. Abth., 1884. Phylogeny of Vertebrate eye, Dohrn, Mitth. Zool. Stat. 

 Naples, vi. 1885. Ear, Retzius, Gehororgan der Wirbelthiere, Stockholm, i. 



