450 
five myotomes which give rise to the M. par. subbranchialis 
in Petromyzon are the exact homologues of those myotomes which 
in S. acanthias produce the “hypoglossus musculature” is a question 
which I now propose to discuss. In order to do this it will be 
necessary briefly to describe the development of the hypoglossus 
musculature in S. acanthias. 
Development of the “Hypoglossus Musculature” 
in Squalus acanthias. 
VAN WHE (92) was the first to study the development of the 
hypoglossus musculature in Selachii. He found that in Pristiurus 
embryos four myotomes, viz. those of his 7th—10th (8rd—6th post-otic) 
somites inclusive, do not enter into the anterior pair of appendages 
but grow ventrad and forward below the gill clefts. Cornına (95) 
counted in Scyllium embryos five buds which form this musculature. 
Buds from the nine myotomes behind these enter into the formation 
of the pectoral plate. Also, in Teleostean and Reptilian embryos the 
five anterior myotomes form the hypoglossus musculature. 
I find that in embryos of Squalus also buds from five myotomes 
form the Anlage of the hypoglossus musculature, and so far I am 
able to confirm Cornina’s results. These buds are, however, not in 
Squalus derivatives of the first five post-otic myotomes. In an embryo 
of 16 mm there lie anterior to the first myotome which has a ventral 
growing bud, two!) myotomes, the anterior of which is very rudimen- 
tary, while the posterior is well developed and possesses a (somewhat 
rudimentary) ventral nerve root. These two anterior myotomes are 
those of van WisHE’s 6th and 7th somites (2nd and 3rd post-otic) ?) 
and, as I believe, the homologues of Ragr's Ist and 2nd “Gesamt- 
segmente”. In this stage of development, buds from the following 
myotomes are well differentiated, those from the 4th—6th myotomes 
being considerably the longest. Gradually as the buds elongate their 
connection with the myotome becomes reduced to a hollow stalk whose 
diameter equals that of the ventral nerve root (20—22 mm embryos). 
In embryos of 25—30 mm, the buds from the 4th to the 8th (inclusive) 
post-otic myotomes have lost their connection with these myotomes 
and have come to lie ventral to the branchial basket. They have 
1) Moutrer (95) finds in an embryo of Lacerta muralis with 28 
somites, one myotome anterior to the five which form by their ventral 
growth the hypoglossus musculature. 
2) The rudimentary myotome of the 1st post-otic myotome (van WIJsHE’s 
5th) has disappeared in earlier stages. 
