Alexander Onufrievich Kovalevsky (Russian: Алекса́ндр Ону́фриевич Ковале́вский, 7 November 1840 in Vorkovo, Vitebsk Governorate, Russian Empire – 1901, St. Petersburg, Russian Empire), was a Russian embryologist, who studied medicine at the University of Heidelberg and became professor at the University of St Petersburg.
He was the brother of paleontologist Vladimir Kovalevsky, and the brother-in-law of the mathematician Sofya Kovalevskaya.
(a chordate), B. Larval tunicate
, C. Adult tunicate. Kovalevsky saw that the notochord
(1) and gill slit (5) are features shared by tunicates and vertebrates.
Kowalevsky showed that all animals go through a period of gastrulation.
Kovalevsky discovered that tunicates were not molluscs, but that their larval stage had a notochord and pharyngeal slits, like vertebrates. Further, these structures developed from the same germ layers in the embryo as the equivalent structures in vertebrates, so he argued that the tunicates should be grouped with the vertebrates as chordates. 19th century zoology thus converted embryology into an evolutionary science, connecting phylogeny with homologies between the germ layers of embryos, foreshadowing evolutionary developmental biology.