The accuracy of Krohn's observations concerning the 

 structure of the larvae of gastropodan molluscs is noted in 

 recent books on comparative embryology, For example, 

 K. N. Davydov wrote that "Already, long ago, zoologists turned 

 their attention to the development of gastropodan molluscs, 

 and Krohn and Nordmann left for us memoirs which even at 

 present do not lose their significance" (p. 625) .62 ^ n( j 

 elsewhere: "It is known that the classical case of heteropods 

 (_Firzolid.es) was described by Krohn in 1860" (p. 651). 



A significant place in Krohn's scientific heritage is 

 occupied by his investigations on the development of 

 echinoderms; they were described in no less than ten special 

 reports and transitional notes in works describing other 

 subjects. His systematic study of the embryology of echinoderms 

 Krohn began in 1848 at the time of his three-month stay in 

 Nice. In February-April he experimented with the artificial 

 insemination of the ova of the echinoid Echinus lividus . His 

 observations were compared with data published shortly before 

 by Derbes, 63 w ho also studied the development of ichinus 

 brevispinosus . 



The mature ovum of Echinus lividus, according to Krohn, 

 is covered by membrane (chorion) and composed of yolk (this 

 term Krohn called the ooplasm with nutritional inclusions), 

 with an embryonic vesicle (nucleus) and embryonic spot 

 (nucleolus) . Derbes assertion that the embryonic vesicle 

 disappears in the mature ova up to fertilization, Krohn 

 considered the result of Derbes' insufficiently thorough 

 investigation. Within approximately half an hour after ferti- 

 lization of the ova, the, membrane is separated from the yolk, 

 a phenomenon, which Krohn explained by processes of endosmosis 

 and exosmosis: "the ovum membrane absorbs the fluid from the 

 surroundings and again gives it, to its internal surface" 

 (p. 6). After fertilization the embryonic vesicle and the 

 embryonic spot are no longer seen. In the place of the ovum 

 nucleus, not far from the surface of the ovum, Krohn saw an 



62. Davydov, TPAITE D ' EMBRYOLOGIE . 



63. Derbes, "Observations sur le mecanisme et les 

 phenomenes qui accompagnent la formation de l'embryon 

 chez l'Oursin comestible," ANN. SC . NAT., 3 Ser., 



8 (1847) , pp. 80 - 98. 



64. a. Krohn, BEITRAG ZUR ENTWICKELUNGSGESCHICHTE DER 

 SEEIGELLARVEN (Heidelberg, 1849), 36 pp. 



575 



