250 ADAM SEDGWICK. 



moreover the posterior limits of the third cannot be made out 

 in Stage I. 



Uohrn,^ however, in his fifteenth study describes a complete 

 mesodermal segmentation as occurring in Torpedo mar- 

 morata at a stage in which the mandibular and hyo- 

 branchial pouches could be made out. The embryos in ques- 

 tion were considerably younger than the embryos in which 

 V. Wyhe first observed the segmentation of the cranial meso- 

 derm, and Dohrn ascribes them to Stage F ; but the above- 

 named pouches being present, he was able to compare his 

 cephalic myotomes with those of Wyhe. He makes out ten 

 myotomes in front of the hyoid pouch, arranged as follows : 



4 myotomes in the place of Wyhe's first. 



3 „ „ „ second or mandibular. 



3 „ „ „ hyoid. 



2 or 3 „ „ „ fourth. 



He admits that they are very transitory structures, and that 

 they have lost their distinctness (by fusion with one another) 

 in Stage G, i. e. before the stage at which v. Wyhe first saw 

 them. Having a very practical acquaintance with the great 

 variation of the mesoderm in embryos of different genera of 

 Elasmobranchs I do not venture to impugn the accuracy of 

 Dohrn's observations on a genus which I have not examined ; 

 but knowing the extreme diflBiculty of satisfactorily observing 

 these rudimentary cranial somites, even when they are un- 

 doubtedly present, I cannot help feeling that it is desirable 

 that Dohrn's statements should receive some confirmation. 

 This confirmation is, to a certain extent, supplied by Herr 

 Killian's^ recently published work on Torpedo ocellata. 

 I say " to a certain extent,'^ because Killian's list of somites 

 does differ slightly from that of Dohrn. I think that it is 

 possible, and I trust that Dr. Dohrn (and Herr Killian) will 

 forgive me for making the suggestion, that he has been misled 

 by deceptive appearances afforded by the somites at the time 

 of their disappearance. I know very well that in looking 



' 'Mittheil. a. d. Zool. Station zu Neapel,' Bd. ix. 

 '■' ' Anat. Anzeiger,' Erganzungsheft, 1891. 



