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Embryonic History: Tree Growth.—A tree grows longitudinally through the activity of a permanently embryonic (meristematic) Tissue situated at the apex of each root and shoot. This embryonic activity comprises three overlapping phases: cell division, cell elongation, and cell maturation. The combined effect of this activity is an increase in the length of root and shoot.
RECAPITULATION, or BIOGENETIC LAW, in zoology, terms applied to the parallel which exists between the embryonic development of an individual animal (ontogeny) and the historical evolution of its race (phylogeny). This parallel is explained by the theory of evolution (q.v.), according to which, in the words of Sidgwick, ((the developmental history of the individual appears to be a short and simplified repetition, or in a certain sense a recapitulation, of the course of development of the species.
Synopsis of the Subclasses.—Primitive reptilian types (Cotylosauria) first appeared in the vast coal measures of the carboniferous at a time when amphibians w-ere at their zenith. The skeleton of one of these amphibia (Seymoitru) exhibits so many reptilian characteristics that, in the absence of information regarding its embryonic history, its relegation to the Amphibia is open to question. A cotylosaur may be recognized by the presence of swollen neural arches and the absence of temporal apertures (fenestrae) in the cranium. |
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