Table 2.

Salient features of notochords compared among amphioxus, tunicates, and vertebrates

	Amphioxus larvae and adults	Tunicate larvae	Vertebrate embryosa
Cell types	I. Discoidal cells (stacked like coinsb; each cell containing transverse myofilaments) II. Müller cells: sparsely distributed; no known function	Early larva: discoidal cells (stacked like coinsb; no myofilaments) Late larva: above cells change to squamous epithelium around fluid-filled lumenc	I. Inner core cellsd: each with a large vacuole; no myofilaments II. Surrounding epithelial cell layer
Extracellular sheath	Inside to out: external laminae, circular collagen layer, and longitudinal collagen layerf	External laminae	Inside to out: external laminae, circular collagen layer, and longitudinal collagen layerf
Organizer genes involved in notochord formationg	Comparable to those of vertebrates	Highly divergenth	Comparable to those of amphioxus
Hedgehog from notochord involved in patterning central nervous system	Yesi	Noj	Yes

^aAlso in adults of more basal vertebrates (e.g., cyclostomes, sturgeons)

^bDuring notochord formation, this arrangement is attained rapidly by inconspicuous cell movements [18, 78]; the more marked cell migrations and intercalations establishing the vertebrate notochord have been termed convergent extension [19]

^cTransitional stages described for ascidians in [18]; there is a similar arrangement in adult appendicularian tunicates [20]

^dOften considered a kind of cartilage [15]; the inner core cells are embedded in an extracellular matrix that is disconcertingly scanty for cartilage, although including some macromolecules characteristic of that tissue [21]

^eAlso sometimes termed the basal lamina or the elastica interna

^fAlso sometimes termed the elastica externa

^gDiscussed in [22]

^hDiscussed in [113]

ⁱDiscussed in [23]

^jDiscussed in [24]