Krohn's work represents the first systematic description 

 of ascidian development in world literature; it remains incom- 

 plete and not free from mistakes, which, of course, does not 

 reduce the historical significance of this undoubtedly- 

 remarkable investigation. But the exact and detailed study 

 of the embryology of ascidians belongs to A. 0. Kovalevsky. 

 In his work, their relationship to the vertebrates was proved, 

 delivering a fatal blow to the metaphysical theory of types 

 in the animal kingdom. It formed the basis of comparative 

 evolutionary embryology, first advanced by Krohn. In the work 

 dedicated to the development of ascidians, 80 Kovalevsky wrote 

 the following: 



Leaving aside the investigations of earlier authors, 

 whose results either are already completely reworked 

 by present scientists, or, to a lesser extent, are 

 partially expanded, we must mention Milne-Edwards, 

 Van Beneden, Kolliker, and, in particular, Krohn. Of 

 all these investigations the results of Krohn's 

 investigations are in closest agreement with our own 

 results. Although he described the accumulation of 

 pigments, which completely coincides with our observa- 

 tions , he did not discover the walls of the saccule in 

 which these organs of sensation are situated, and 

 generally he traced the development step by step. The 

 formation of the axial strand in the tail of the 

 ascidian larva was observed by Krohn, although he 

 explained it as a formation of emptiness in the cells . 

 Although the transformation of the larva into the 

 sessile form was described by him in detail, he had 

 only a slight understanding of the anatomy of the larva 

 and therefore he could not observe the particular 

 features, (p. 41) 



At the end of the 1860s, in Naples, Krohn studied budding 

 in the complex ascidian Botryllus , and he presented the results 

 of his observations in two reports. 81 In the first of these 



80. A. O. Kovalevsky, "Istoriya razvitiga prostykh astsidii" 



(The history of development of the simple ascidia) (1886) , 

 SELECTED WORKS (Izd. AN SSSR, 1951) , pp. 41 - 78. 



81. A. Krohn, "Uber die fruheste Bildung der Botryllusstocke," 

 ARCH. NATURG., 35 (1869), pp. 190 - 196; Uber die 

 Fortpflanzungsverhaltnisse bei den Botrylliden, " ibid ., 

 pp. 326 - 333. 



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