426 F. H. Krecker, 



tlie cells point to such a derivation for some of them. However, mucli 

 of this dorsal mesoblast appears to come by migration from tlie iieo- 

 blasts of the ventral side. Cross sections taken not far from the posterior 

 end of tlie body sliow large cells of undoubted neoblastic origin applied 

 to tbe walls of the dorsal blood vessel and also on the dorsal mesentery 

 and these are evidently in process of migration to the dorsal side. 

 (Fig. 15 and 16). Frequent cell divisions indicate that they give rise 

 to some of the smaller cells found in the dorsal mesoblast, many of 

 which appear to have had such a dervation. 



The origin of the circular muscles, particularly in Tubifex and 

 Lutnhriculus, has not yet been agreed upon by investigators. Most 

 workers suppose that these muscles arise from the ectoderm, an 

 opinion that is based partly on inference and partly on actual Obser- 

 vation. BÜLOW was unable to determine whether they arose from the 

 ectoderm or the mesoderm but he considered it most probable that 

 they came from the mesoderm. Hepke (97) in Nais elinguis has this 

 to say on the subject. »Die Ringmuskelfasern entstehen gleichfalls 

 aus dem Ectoderm, nachdem die Abschnürung der Neuralanlage statt- 

 gefunden hat, und zwar auf die Weise, daß einzelne Zellen aus dem 

 Ectoderm in das Innere der Leibeshöhle treten, sich an die Innenfläche 

 derselben anlegen und quer zur Längsachse des Tieres in lange Muskel- 

 zellen auswachsen.« Abel is of somewhat the same opinion with 

 regard to Tubifex, mainly because he saw the circular muscle closely 

 applied to the ectoderm. The chief aim of his paper, however, was 

 not histogenesis. Iwanow (03), who worked on Lumbriculus, frankly 

 says, »Meine eignen Untersuchungen führten nicht zur Lösung der 

 Frage über den Ursprung der Ringmuskulatur der Leibeswand«, but 

 in his endeaver to attribute all mesodermal structures to the neoblasts 

 he is inclined to think that the circular muscles come from the somatic 

 layer of the coelomic sacs. What v. AVagner has to offer on the subject 

 is merely that it is »höchstwahrscheinlich« that the circular muscles 

 come from the ectoderm, »nämlich von Elementen, die von den Zellen- 

 nestern herrührenden subepidermoidalen Zellenschicht (Dermoblasten) 

 gebildet werden«. He too studied Lumbriculus. From the quotations 

 given it is seen that the origin of the circular muscles in regenerating 

 oligochaetes is still an open question and a difference of opinion is also 

 evident in the accounts of its embryological origin. Bergh (90) and 

 Vejdovsky (83) think they arise from the ectoderm independently of 

 the mesoblastic bands. Roule believes they come from the mesoblast, 

 and Wilson (89) says that all the mesodermal structures come from 



