No. I.] GERM-LAYERS IN CLEPSINE. 145 



quantity, it is evident that Bergh has made a closer approxima- 

 tion to precision than any of his predecessors. Grube is the 

 least definite of all; Robin (No. 5, pp. 199, 344) regarded the 

 entire germ-bands as the central nervous system. Rathke and 

 Leuckart traced its origin to the median portion of the bands ; 

 Metschnikoff limited it to a single stratum, and Bergh to the 

 median portion of the same stratum. 



From the following comments by Bergh it will be seen that 

 he thought it impossible to trace the nerve-cord to special 

 neuroblasts in Aulostoma, and a little more than impossible ( !) 

 in the case of Clepsine : — 



" Whitman hat die zehn Zellen am Hinterende der Rumpf- 

 keime gefunden, welche bei Clepsine durch ganz besondere 

 Grosse ausgezeichnet sind ; die acht derselben nennt er Neuro- 

 blasten, die zwei dagegen Mesoblasten, indem er annimmt, dass 

 aus den ersteren nur Nervensystem, aus den anderen nur Meso- 

 derm entstehe. Vergeblich sucht man in der genannten Arbeit 

 ebenso wie in der Natur selbst irgend einen Beweis fiir diese 

 Behauptung, die eben nur eine solche ist. Es ist bei Aulostoma 

 vollkommen unmoglicJi, die Descendenten jeder einzelnen der 

 erwahnten grosseren Zellen fiir sich zu verfolgen, und bei 

 Clepsine wird die-Sache noch viel schwierigcr, indem die Rumpf- 

 keime hier stark gekriimmt sind. Richtig ist es aber, wenn 

 Whitman ganz im Allgemeinen die Bauchkette aus den Rumpf- 

 keimen herleitet" (No. 9, p. 259). 



In Bergh's Fig. 23^, PI. XV., all the strata of the germ-bands 

 are clearly defined ; but if we compare this Fig. with Fig. 2413; 

 and b, the entire neuro-nephric stratum appears to be employed 

 in the formation of the " definitive epidermis." On page 263 

 the nerve-cord is said to arise beneath the " Anlage der defini- 

 tiven Rumpfepidermis." This statement, taken in connection 

 with the illustrations, of PI. XV., would lead one to suppose that 

 the nerve-chain had its origin in the inner (mesoblastic) layer 

 of the germ-bands. Scarcely more definite are his descriptions 

 in the case of Nephelis (No. 13, p. 295, PI. XIX.). All the 

 strata are clearly represented in the figures, but the neuro- 

 nephric stratum is marked ep ("definitive epidermis"), and 

 there is not the slightest indication of any distinction between 

 neural and epidermal elements. The origin of the nervous 

 system from this layer is conceded in a foot-note (p. 295), in 



