NOTES ON ELASMOBRANCH DEVELOPMENT. 575 
moreover the posterior limits of the third cannot be made out 
in Stage I. 
Dohrn, 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 
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 asfollows: 
4 myotomes in the place of Wyhe’s first. 
3 5 3 » second or mandibular. 
3 ey) oy) » hyoid. 
DOr oi. i 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 difficulty 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* receutly 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. 1 know very well that in looking 
1 «Mittheil. a. d. Zool. Station zu Neapel,’ Bd. ix. 
2 * Anat. Anzeiger,’ Erganzungsheft, 1891. 
