453 
stages of development — before what may be regarded as the primitive 
segmental relations are greatly changed — leads me to the conclusion 
that the somite of the first myotome of the lateral 
muscle of Petromyzon is the exact homologue of van 
Wısne’s 5th, and not his 4th somite in Acanthias. I now 
pass to the consideration of the evidence which leads me to this 
conclusion. 
I have found in a previous study of neuromeric segmentation (’96), 
that in different groups of Vertebrates the otic invagination takes 
place opposite the neuromere which I have termed encephalomere VI. 
I believe therefore we may safely assume that the otic invagination 
takes place in segments exactly homologous in Petromyzon and Squalus. 
May we then homologize the first post-otic somites in these two forms ? 
The first post-otic somite in Squalus is van Wurne’s 5th, which is 
connected with the mesoderm of the 3rd branchial arch. The results 
of investigators as to the primitive relations of the first post-otic 
somite in Petromyzon are conflicting. Scorr (80, ’82, 87) and 
SHIPLEY (’87) agree that the first pair of clearly differentiated somites 
follow close behind the otic capsule, instead of leaving a considerable 
interval free. KUPFFER, however, has stated evidence in several 
publications which directly contradicts this. He writes (96, p. 613) 
that “bald nach dem Ausschliipfen, wenn die jungen Ammocoeten etwa 
eine Länge von 3,5 mm aufweisen, besteht hinter dem Labyrinthe ein 
regelmäßig gestaltetes Myotom, das jetzt erste metaotische. Es ist 
um etwa seine doppelte Länge vom hinteren Pol der Labyrinthblase 
entfernt. Das zweite Myotom ist breiter als das erste.” He finds 
that as the result of the budding of the second meta-otic myotome the 
definitive M. lateralis capitis anterior (Kuprrer) is formed. 
My observations confirm those of Scorr and SmipLey. In embryos 
P. marinus of the 8th—10th days of development, i. e. before hatching, 
and in embryos of P. Planeri of 2,8 mm to 3,5 mm, the series of 
somites ends opposite the middle of the otic capsule. The two anterior 
somites are cleft longitudinally into a median and a lateral portion‘), 
of which the median terminates between the otic capsule and the 
chorda and the lateral portion, between capsule and ectoderm. In a 
4 mm embryo of P. Planeri (or 11th day embryo of P. marinus) the lateral 
dortion of the first somite is seen to have extended forward lateral 
1) The IX and X nerves pass ventrally between these two por- 
tions (median and lateral) of the first two somites. Of the important 
bearing of this fact upon the theories as to the morphology of the cranial 
nerves I shall speak in a forthcoming paper. 
