No. I.] GERM-LAYERS IN CLEPSINE. jcr 



it is really entitled to. That it is not based upon sufificiently 

 thorough observations is evident enough from Kowalevsky's 

 own words. " Die Embryonen der Hirudineen," says Kowa- 

 levsky (No. lO, pp. 1-2), " eru'iesen sich aber zu Querschnitten 

 nicht ganz passend, und es gelang inir nur mit grosster Miihe, 

 einige feme QiierscJmitte ansufertigeji, auf welcken ich die 

 Scheidung in Keimbl'dtter uitd die Bildung des Nervensystems 

 aus dcm oberen Blatte sehen konnte, aber nicht den Keimstreifen, 

 aus dem sich lediglich die Muskeln entwickeln." 



In regard to Clepsine, Kowalevsky remarks (p. 3) : " Da ich 

 aber zur Zeit der Entwickelung derselben gerade mit den Ac- 

 cipensern beschaftigt war, so bev/ahrte ich jtttr mehrere Stadien 

 in schwacher CJiromsaure aiif ; an Querschnitten derselben k'onnte 

 ich mich sp'dtcr nur von der Abstammung des Nervensystems 

 voin oberen Blatte ilberzeiigen." 



Kowalevsky's conclusion as to the origin of the nerve-system 

 rests, then, as he himself acknowledges, on the study of a few 

 eggs so imperfectly preserved and prepared that it was im- 

 possible to understand the structure of the germ-bands. How 

 Kowalevsky convinced himself, with such material as this, that 

 the nerve-chain arises from the epidermal layer, must be left 

 entirely to conjecture ; for not a word of explanation is offered, 

 nor a single figure given. 



Semper (No. 18), in his well-known work on ''Die Ver- 

 wandtschaftsbeziehungen der gegliederten Thiere,'^ reviews the 

 statements of Rathke, Leuckart, Kowalevsky, and Metschnikoff 

 on the origin of the nerve-system in the Hirudinea, and en- 

 deavors to reconcile them with his own observations on Nephe- 

 lis, Nais, Chaetogaster, etc. "Nach eigenen Untersuchungen," 

 says Semper, " kann ich fiir Nephelis die Angaben Rathke's 

 bestatigen, dass die Ganglien entstehen durch Sonderung der 

 medialen Parthien des Keimstreifens ; cine mittlere, von diesem 

 unabhdngige EctoderrnverdickuJig tritt bei dieser Gattung so we- 

 nig, wie bei Chaetogaster ein " (p. 246). 



Semper, however, maintains that the nerve-chain has a double 

 origin, its median portion being derived directly from the ecto- 

 derm, and the lateral portions (ganglia) from the mesodermic 

 layer of the germ-bands. This view enables him to explain the 

 contradictory results of different writers. Metschnikoffs state- 

 ments regarding Clepsine are commented on as follows (p. 178) ; 



