Ontogeny does not, in general, recapitulate phylogeny. The other two types are advanced. Zahur regards that the first type, where the companion cell is much shorter than the accompanying sieve tube, is primitive.
Evolution , 37 , 861—3. The University of Massachusetts Press, Amherst. Some biologists took up the idea with enthusiasm, pointing to examples of recapitulation throughout embryology; others argued that resemblances between early-stage embryos reflect not ancestral adult forms but the need of all organisms to achieve their diverse forms starting from a single cell, the fertilized egg.
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If the next stage in the evolution of mammals was amphibian, all mammal embryos should next resemble amphibians. Yet the embryos of some species of baleen whales develop teeth and re-absorb them before birth, strong evidence that baleen whales evolved from ancestors who did have teeth as adults. The nucleus disappears or may persist as collapsed body. The process described by the biogenetic law is referred to as recapitulation.
Sachs, J. The earliest stages of individual development often differ greatly, but there is a stage, the zootype or phylotypic stage, at which all vertebrate embryos do bear a general resemblance to each other.
Procambium gives rise to phloem parenchyma of primary phloem.
For example, in all vertebrates the first pharyngeal arch forms the jaw. Retrieved February 27, 2019 from Encyclopedia. Schell , Academic Press, New York, pp.
Later there appear the plasmodesmata, which mark the site of future sieve area. Planta , 105 , 33—42. Umrath, K. Goodwin, B. The parent cell divides by unequal longitudinal division and the smaller cell develops into companion cell. Notes R.
February 27, 2019. Numerous, independent, convergent, ever-more-detailed lines of evidence prove that all species have descended from shared ancestors. Sieve cell predominates in pteridophyte and gymnosperm.