netted TREY SU iS 
1385] Fhysiology. | 97 
abruptly as if cut or bitten off, the hinder hypaxial and epaxial 
tissues concerned in the formation of rays and their supports are 
then approximated over the end of the aborted axis so as to form 
a continuous chain, and developed later than the other and more 
anterior median fin-rays (Mola), and the interval so bridged by a 
secondary process of development leads to the formation of what 
we may call a gephyrocercal tail, in which the spinous axial 
apophyses of the caudal vertebra, together with their centra, fail 
to develop, and the caudal rays rest either upon interspinous ele- 
ments alone, or even these may be almost entirely aborted, as is 
the case for a time in the young stages of Mola “ Ostracion boops No 
and “ Molacanthus,” both of which are evidently young, post- 
larval phases of that form. 
The views here outlined rest partly upon facts of my own 
observation, but I must express my great indebtedness to the 
researches of L. and A. Agassiz,Vogt, Lotz, Balfour, Parker, Hux- 
ley and Kölliker, whose labors have enabled me to coordinate 
the facts and establish doctrines respecting the origin of the 
median fins, which are founded upon the theory of ontogeny. — 
John A. Ryder. Nov. 3d, 1884. ` 
EXPLANATIONS OF FIGURES. 
Fic. 1,—Larval Branchiostoma, (after Kowalevsky); almost perfectly archicercal. 
Fic. 2.—Chimera monstrosa, with an archicercal opisthural filament, (after: 
Agassiz). 
Fic. 3.—Lophocercal larva of the codfish, with continuous median fin-fold, f f f f- 
Fic, 4.—Ideal diphycercal tail, nearly as in Ceratodus and Protopterus. 
Fic. oe archaic heterocercal tail; somewhat as found. in sturgeons andi 
sharks. 
Fic. 6.—Heterocercal tail of larval Lepidosteus (after Balfour and Parker), showing 
epural and hypural pieces undeveloped at the end of the chorda. 
Fic. 7.—Tail of a very young Lepidosteus (from the same source), showing the 
opisthure, of, above the secondary or true caudal, sc. 
Fic. 8.—Caudal skeleton of a larval Amiurus, fifteen days old. o, opisthural, Ay, 
hypural, and cf, epural cartilages; x, urostyle ; mt, medulla spinalis ; cå, chorda, 
invested by the skeletal tissue, så, of the caudal vertebræ. 
PHYSIOLOGY.! 
Tue THERAPEUTIC EFFECTS OF OXYGEN AND OF Ozong.—It is 
re. 2. In cases of poisoning with chloroform, alcohol, sula — 
phuretted hydrogen or carbonic oxide, respiration of pure oxy- 
‘This department is edited by Professor Henry Sewatt, of Ann Arbor, Mich. 
= YOR XIX.—No. I. 7 
