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 meanwhile appear at the sides of the axial notochordal tract, the mesoblastic somites. The first of these differs in several respects from those which succeed, and has been called the collar cavity (MacBride). In front of the latter there remains a portion of the archenteron, which becomes constricted off as the head cavity. This becomes divided into two, the right half forming the cavity of the rostrum, while the left acquires an opening to the exterior, and forms the praeoral pit of the larva, which subsequently gives rise to special ciliated tracts in the vestibule of the mouth mentioned above. The larval period commences at about the thirty-sixth hour with the perforation of the mouth, first gill-cleft and anus. The larva is curiously asymmetrical, as many as fourteen gill-clefts appearing in an unpaired series on the right side, while the mouth is a large orifice on the left side, the anus being median. The adult form is achieved by metamorphosis, which cannot be further described here. One point must not be omitted, namely, the homogeny of the endostyle of Amphioxus and the thyroid gland of Craniata.

AMPHIPOLIS (mod. Yeni Keui), an ancient city of Macedonia, on the east bank of the river Strymon, where it emerges from Lake Cercinitis, about 3 m. from the sea. Originally a Thracian town, known as  (“Nine Roads”), it was colonized by Athenians with other Greeks under Hagnon in 437, previous attempts—in 497, 476 (Schol. Aesch. De fals. leg. 31) and 465—having been unsuccessful. In 424 it surrendered to the Spartan Brasidas without resistance, owing to the gross negligence of the historian Thucydides, who was with the fleet at Thasos. In 422 Cleon led an unsuccessful expedition to recover it, in which both he and Brasidas were slain. The importance of Amphipolis in ancient times was due to the fact that it commanded the bridge over the Strymon, and consequently the route from northern Greece to the Hellespont; it was important also as a depot for the gold and silver mines of the district, and for timber, which was largely used in shipbuilding. This importance is shown by the fact that, in the peace of Nicias (421, its restoration to Athens is made the subject of a special provision, and that about 417, this provision not having been observed, at least one expedition was made by Nicias with a view to its recovery. Philip of Macedon made a special point of occupying it (357), and under the early empire it became the headquarters of the Roman propraetor, though it was recognized as independent. Many inscriptions, coins, &c., have been found here, and traces of the ancient fortifications and of a Roman aqueduct are visible. AMPHIPROSTYLE (from the Gr. , on both sides, and  , a portico), the term for a (q.v.) with a portico both in the front and in the rear. AMPHISBAENA (a Greek word, from  , both ways, and  , to go), a serpent in ancient mythology, beginning or ending at both head and tail alike. Its fabled existence has been utilized by the poets, such as Milton, Pope and Tennyson. In modern zoology it is the name given to the main genus of a family of worm-shaped lizards, most of which inhabit the tropical parts of America, the West Indies and Africa. The commonest species in South America and the Antilles is the sooty or dusky A. fuliginosa. The body of the amphisbaena, from 18 to 20 in. long, is of nearly the same thickness throughout. The head is small, and there can scarcely be said to be a tail, the vent being close to the extremity of the body. The animal lives mostly underground, burrowing in soft earth, and feeds on ants and other small animals. From its appearance, and the ease with which it moves backwards, has arisen the popular belief that the amphisbaena has two heads, and that when the body is cut in two the parts seek each other out and reunite. From this has arisen another popular error, which attributes extraordinary curative properties to its flesh when dried and pulverized. AMPHITHEATRE (Gr. , around, and  , a place for spectators), a building in which the seats for spectators surround the scene of the performance. The word was doubtless coined by the Greeks of Campania, since it was here that the gladiatorial shows for which the amphitheatre was primarily used were first organized as public spectacles. The earliest building of the kind still extant is that at Pompeii, built after 80 It is called spectacula in a contemporary inscription. The word amphitheatrum is first found in writers of the Augustan age.

In Italy, combats of gladiators at first took place in the forums, where temporary wooden scaffoldings were erected for the spectators; and Vitruvius gives this as the reason why in that country the forums were in the shape of a parallelogram instead of being squares as in Greece. Wild beasts were also hunted in the circus. But towards the end of the Roman republic, when the shows increased both in frequency and in costliness, special buildings began to be provided for them.

The first amphitheatre at Rome was that constructed, 59, by C. Scribonius Curio. Pliny tells us that Curio built two wooden theatres, which were placed back to back, and that after the dramatic representations were finished, they were turned round, with all the spectators in them, so as to make one circular theatre, in the centre of which gladiators fought; but the story is incredible, and must have arisen from the false translation of  by “double theatre.” It is uncertain whether Caesar, in 46, constructed a temporary amphitheatre of wood for his shows of wild beasts; at any rate, the first permanent amphitheatre was built by C. Statilius Taurus in 29 Probably the shell only was of stone. It was burnt in the great fire of 64.

We hear of an amphitheatre begun by Caligula and of a wooden structure raised in the year 57 by Nero; but these were superseded by the Amphitheatrum Flavium (known at least since the 8th century as the Colosseum, from its colossal size), which was begun by Vespasian on the site of an artificial lake included in the Golden House of Nero, and inaugurated by Titus in  80 with shows lasting one hundred days. It was several times restored by the emperors, having been twice struck by lightning in the 3rd century and twice damaged by earthquake in the 5th. Gladiatorial shows were suppressed by Honorius in 404, and wild beast shows are not recorded after the reign of Theodoric (d.  526). In the 8th century Bede wrote Quamdiu stabit Coliseus, stabit et Roma; quando cadet Coliseus, cadet et Roma. A large part of the western arcades seem to have collapsed in the earthquake of 1349, and their remains were used in the Renaissance as a quarry for building materials (e.g. for the Palazzo di Venezia, the Cancelleria and the Palazzo Farnese).

Rome possesses the remains of a second amphitheatre on the Esquiline, called by the chronologist of 354 Amphitheatrum Castrense, which probably means the “court” or “imperial”