444 
fore mesodermal in origin — and that it is homologous with the 
hypoglossus musculature of Gnathostomata. 
In embryos of the eleventh day, 5!/, mm, cellular buds are seen 
to extend ventrad from those myotomes which lie directly behind the 
branchial region (Fig. 1). These differ from such myotomic buds as 
one finds in Selachian embryos chiefly in the fact that they possess 
no lumen. The cells of the buds are loose, and they stain more 
deeply than the cells of the parietal layer ot mesoderm and the ecto- 
derm between which the buds lie. In some embryos of the tenth 
day of development I find little or no evidence of such ventral growth 
of the myotomes, while in others the buds appear, but their number 
is less and they extend less far ventrad than those seen in embryos 
of the eleventh day. In all cases the anteriormost bud develops from 
the 7th permanent, or post-otic, myotome, i. e. the first myotome 
behind the branchial region). Behind this, there extend from the 
myotomes buds, whose size diminishes progressively as one passes 
posteriorly. The buds from the 6th 7th and 9th myotomes are 
In vag. 
ep. vag - 
in. b Mm. pac vent 
Fig. 1. Diagram illustrating the development and relations of the hypoglossus mus- 
culature of Pet. marinus, as seen in embryos of the 11th—13th days of development. 
1—10 post-otic myotomes; /1— 5! visceral pouches; aw. otic capsule; ep. vag. epi- 
branchial portion of vagus; gn. fac. facialis ganglion; gn. gls. glossopharyngeus ganglion; 
gn. trgi—* first and second trigeminus ganglia; gn. vag. vagus ganglion; M. mouth; 
my. b! first myotomie bud; m. par. vent. Anlage of the ventral longitudinal muscle of 
the branchial region (13th day); N. nasal invagination; n. dat. nervus lateralis; ‘“ Vag.- 
Anhang” the ganglion of the nerve called by HATSCHEK (’92) ‘“spinal-artige Vagus- 
Anhang”. 
1) Below this myotome, as Razz (’96) has stated, is found the first 
pronephric tubule. W. Müuzer (75) found four, FÜrBRINnGER (’78) five, 
and Rast (’96) four pronephric tubules in Petromyzon. In embryos of 
the 10th day (day of hatching) the number of pronephric tubules varies 
from four to five in Pet. marinus. These lie below the 7th—10th myo- 
tomes inclusive. 
