The article in which this discovery was published possesses 

 important significance in the history of doliolum, as it 

 contains the first observations on the development of this 

 animal. Krohn not only described the tailed larva of doliolum, 

 tut he also pointed out the alternation of generations in the 

 reproduction of this animal.... On the basis of his observa- 

 tions Krohn concluded that from the ovum of doliolum is formed 

 the tailed, freely swimming larva, which after the 

 loss of the tail is transformed into an asexual doliolum" 

 (p. 2). Later on Ulianin noticed Krohn's mistake in dividing 

 doliolum according to the number of muscular strips. In 

 all species of sexual generation there are eight, but in 

 asexual species there are nine strips. Therefore the species 

 described by Krohn, Doliolum Trosoheli , is in fact an asexual 

 individual D. dentioulatum Q. and G., and D. Nordmannz Krohn 

 is the asexual generation of D. Mulleri Krohn. Particular 

 significance was given by Ulianin to Krohn's embryological 

 observations. "All that is known presently about the 

 embryological development of Doliolum," Ulianin wrote, 

 "comes exclusively from Krohn, who for the first time described 

 the free larva. All later authors.... only redescribed it, not 

 adding anything essential to Krohn's description (p. 47). 



The central place among Krohn's investigations of the 

 development of tunicates is occupied by his work concerning 

 the solitary ascidians.? 9 The artificial insemination in 

 ascidians which was used successfully for the first time by 

 Baer, Krohn also used, observing the development of Phallusia 

 mammillata step by step for three months. He described the 

 mature ovum of this ascidian in the following way. The ovum 

 present in the ovum-fluid is supplied by papillae and covered 

 by a cover membrane under which the proper ovum membrane is 

 present. Somewhat deeper lies a hyaline membrane containing 

 inclusions which is green in color. The yolk itself is 

 colorless, the embryonic vesicle and the embryonic spot in 

 the mature ova are unnoticeable . The above mentioned green 

 hyaline membrane was considered by Krohn, following Milne- 

 Edwards, a source of formation of tunica. The error of this 

 view was later established, but it was repeated in many 



79. A. Krohn, "fiber die Entwickelung der Ascidien," ARCH. 



ANAT., PHYSIOL. (1852), pp. 312 - 333. In the following 

 year this work was published in English as "On the 

 Development of the Ascidians," SC. MEM. NAT. HIST. 

 (1853) , pp. 312 - 329. 



589 



