No. I.] GERM-LAYERS IN CLEPSINE. 141 



keime"). Leuckart (No. 17, p. 706) described the head- 

 bands in Hirudo as " Zwei einfache seitliche Anschwellungen, 

 die rechts und links vor der Mundoffnung gelegen sind und 

 durch eine ziemlich lange Commissur sowohl unter sich, als 

 auch mit den jetzt hornformig ausgezogenen Vorderenden der 

 Unterschlundganglienmasse zusammenhangen." Semper (No. 

 18, p. 215) was the first to point out two distinct head-bands 

 (" Sinnesplatten," " Kopfkeimstreifen ") in the Hirudinea. " Bei 

 Clepsine, wie bei Nephelis, der Schlundring und das dorsale 

 SchlundgangHon entsteht gerade so wie bei Hirudo, durch Ver- 

 wachsen zweier Sinnesplatten." He insists, however, that 

 these sense-plates (p. 247) are " echte Kopfkeimstreifen, von 

 deren Bildungszellmasse nur ein Theil zum Nervensystem wird, 

 wahrend ein anderer Theil sich in die iibrigen Organe des 

 Kopfes, vor Allem in die mit dem Schlunde sich verbindenden 

 Organe umwandelt." 



As above remarked, I am not prepared to discuss the question 

 as to the existence of two independent germ-bands in the 

 cephalic lobe. I have stated (No. i ) my conviction that this 

 lobe is formed at the expense of the first four micromeres. 

 Should there prove to be two head-bands, as maintained by 

 Semper and Bergh, it would be an interesting problem to deter- 

 mine whether they hold the same relation to the first four 

 micromeres as the germ-bands of the trunk to the teloblasts. 

 If such a relation could be demonstrated, the homodynamy of 

 head and trunk would be placed in a very instructive light. The 

 entire embryo, with exception of the mesenteron and epidermis, 

 would then be built up in fundamentally the same manner, at 

 the expense of terminal blastomeres, ten teloblasts, and four 

 micromeres, or acroblasts. The theoretical difficulty presented 

 by independent rudiments for the head and trunk could then be 

 disposed of. The temporary separation of the rudiments might 

 be regarded as an accident of their present mode of origin 

 rather than as an expression of their primitive relations. Their 

 real and essential unity is discoverable in the history of the 

 originating blastomeres. It will be remembered that the pos- 

 terior macromere produces not only the mesoblasts, neuroblasts, 



(17.) Leuckart, R. Die Menschlichen Parasiten. I. 1863. 

 (18.) Semper, C. Die Verwandtschaftsbeziehungen der gegliederten Thiere. 

 Arb. a. d. Zool.-Zoot. Inst, in WUrzburg. III. 1876. 



