461 
in Squalus the visceral pouches crowd forward in development so that 
the 4th and 5th come to meet the ectoderm successively below the 
same somite !) (2nd post-otic). This shoving forward of the branchial 
clefts with reference to the myotomes, permits the ventral growth of 
a myotome, which would be prevented were the branchial pouches 
exactly segmental as in Petromyzon. 
Although the anterior portion of the ventral longitudinal muscle 
of Petromyzon is not composed of segments exactly homologous with 
those which form this muscle in Squalus, I nevertheless believe that 
we may conclude from its development and relations that the muscle 
itself is homologous in these two forms, and throughout the Vertebrates 
series. 
The preceding study has been made in the laboratory of the 
Anatomical Institute in Munich during the winter semester of 1896— 97. 
I wish to express my sense of indebtedness to the staff of that in- 
stitution, especially to its Director, Prof. von Kuprrer, for his kind 
interest in my work, and to Dr. Borum and Dr. Neumayr. My 
abundant material of Petromyzon marinus embryos I owe to the great 
generosity of my friend and former teacher, Prof. E. L. Mark. 
Miinchen, March. 30, 1897. 
List of papers cited. 
84. AutsorRn, F., Ueber den Ursprung und Austritt der Hirnnerven von 
Petromyzon. Zeitschr. f. wiss. Zoöl., Bd. 40, p. 286—308, Taf. 18. 
27. Born, G., Ueber den inneren Bau der Lamprete. Heusinger’s Zeitschr. 
for» Ehys., ‘Bary, 
’91. Busor, P., Contribution a l’6tude de la métamorphose de l’Ammocoetes 
branchialis et Petromyzon Planeri. Rev. biol. du Nord de la France, 
Tome 3, 99 pp., 6 pl. 
’95. Corninc, H. K., Ueber die Entwickelung der Zungenmusculatur bei 
Reptilien. Verh. d. Anat. Ges. in Basel, Bd. 10, p. 165—175. 
’84. Donen, A., Die paarigen und unpaaren Flossen der Selachier. Mitt. 
Zoöl. Stat. Neapel, Bd. 5, p. 161—195, Taf. 9, 
’85. Frortep, A., Ueber Anlagen von Sinnesorganen am Facialis, Glosso- 
pharyngeus und Vagus, über die genetische Stellung des Vagus zum 
Hypoglossus, und über die Herkunft der Zungenmusculatur. Arch, f. 
Anat. u. Entwick., p. 1—55, Taf. 1—2. 
’75. Fissrinerer, P., Untersuchungen zur vergl. Anatomie der Musculatur 
des Kopfskelets der Cyclostomen. Jen.Zeitschr., Bd. 9. p. 1—93, pl. 1—3. 
’94. GEGENBAUR, C., Zur Phylogenese der Zunge. Morph. Jahrb., Bd. 21, 
ps 1218; 
1) A similar phenomenon has been noted in Necturus by Miss 
Pıarr (96). 
