Studien zur Urgeschichte des Wirbelthlerkörpers. 2-1. 227 



Arünment beweg't ilin dann, die folgende Anmerkung zu machen: 

 An examinatiou of some fìnely preserved emì)ryos of Torpedo ocellata, 

 kindly given me by my friend, Prof. A. N. Sewertzoff, leads me 

 to agree with Öedgwick (1892) that this is not true of the meso- 

 derra segments discovered by Dohrn (1890) in that form. Dohrn 

 apparently did not endeavor to ascertain whether they were symme- 

 trica! or not. I am uuable to determine, whether or not there is a 

 correspondence of the mesoderma} segments on the two sides of the 

 head anterior to the one which, in my opinion, corresponds with 

 the 15th Segment of Doiirn. While my own negative conclusions 

 cannot be regarded as in any sense disproof of the segmental value 

 of Dohrn's segments, it is my opinion that the evidence of their 

 variability shown by the conflicting results of Killian tends to 

 throw considerable doubt opon it. Since Killiax finds that of the 

 anterior of these segments one is to be regarded as the sclerotome 

 portion of a somite, while others are simply vesicular enlargements 

 of the mesoderm of the mandibular arch, it is to be inferred that 

 DoHRN subjeeted the head somites of Torpedo to little criticai 

 examination. To regard as evidence of somites all vacuolar Spaces 

 in the dorsal (and lateral!) mesoderm which appear between the 

 somatopleure and splanchnopleure at the time these layers separate, 

 seems to be too uncritical. Similar phenomena appear in the meso- 

 derm of Squalns in those early stages of development, when the 

 coelom is in the process of formation, viz. in stages when the neu- 

 ral piate is widely expanded and the embryo possesses 4 or 5 so- 

 mites. Recent studies by Sewertzoff render still more doubtful 

 the results of Dohrn and Killian«. Wie weit meine erste Dar- 

 stellung der Somitenbildung des Kopfes von Torpedo unrichtig war, 

 habe ich in der 21. Studie pag. 194 selbst ausgesprochen, habe da- 

 bei aber auch meine Reserven gegen die damals vorliegenden 

 Einwürfe von Killiax und Severtzoff gemacht, die NEAL'sche 

 Kritik indess noch nicht berücksichtigt. Um so mehr habe ich 

 jetzt darauf einzugehen und werde es weiter unten thun , so 

 weit Neal mich dessen nicht in eigner Person überhebt. Denn 

 er geht auf pag. 195 dazu über, selbst Argumente aufzustellen, 

 um die Unregelmäßigkeiten und Unvollständigkeit in der Bildung 

 und Ausbildung der vorderen Mesodermbildungen, im Widerspruch 

 gegen Rabl, zu erklären. So sagt er auf pag. 195-. »Differences 

 in time of development and of ditferentiation are to be expected 

 when a comparison is made between the Anlagen of serial organs, 



15* 



