on the summit of the placenta under its cover (138) . In this 

 rudiment of the embryo all organs of the last are developed; 

 "however," Krohn wrote, "all that concerned its development 

 during the first period remained for me almost completely 

 unknown" (p. 123). He could only establish that one of the 

 first organs of the embryo by the time of appearance is "the 

 respiratory cavity." The embryo changes from compact to hollow, 

 following which the rudiments of the branchae and nervous 

 ganglion are already seen, while the organs "visceral nucleus" 

 and heart become noticeable only later. Only after that does 

 the embryo acquire a definite form; the anterior and posterior 

 openings appear in it. At the end the embryo becomes more 

 voluminous than the placenta, and all its organs intensively 

 enlarge, especially the nervous ganglion, from which numerous 

 nervous branches grow. At the same time muscular strips and 

 blood vessels appear, which are not completely formed. 



Krohn' s concluding paragraph of the work is dedicated 

 to the processes of budding in the asexual regeneration of 

 Salpa, and to the formation of colonies; the character of the 

 last varies in different species. Here the description of the 

 stolon and the embryos is given, situated along it so that 

 their axes cross the stolon at a right angle. These embryos 

 develop in definite succession, depending on their situation 

 on the stolon. 



The development of salpa, especially the formation of 

 their embryos from fertilized ova, represents one of the most 

 difficult principles of embryology. Krohn' s investigations 

 began this study and recent opinions are credited to many 

 Russian embryologists at the end of the nineteenth and the 

 beginning of the twentieth century, including A. 0. Kovalevsky, 

 M. M. Davydov, A. A. Korotnev, and V. V. Zalensky. 74 



Six years after the publication of the above-mentioned 

 work, Krohn published an article presenting the results of 

 his investigations of the little-studied group of tunicates, 

 the doliolum.75 Q U oy and Gaimard made a voyage on the 



74. "The first investigations of the development of Salpa go 

 back to. Krohn," K. N. Davydov wrote in his handbook 

 CTRAITE, p. 867) . 



75. A. Krohn, "Uber die Gattung Doliolum und ihre Arten," 

 ARCHIV. NATURG., 18 (1852), pp. 53 - 65. 



586 



