STRUCTURE AND DEVELOPMENT OE LEP1DOSTEUS. 
39.9 
Part III .—The ribs of Fishes. 
The nature and homologies of the ribs of Fishes have long been a matter of contro¬ 
versy ; but the subject has recently been brought forward in the important memoirs 
of Gotte* on the Vertebrate skeleton. The alternatives usually adopted are, roughly 
speaking, these :—Either the haemal arches of the tail are homologous throughout 
the piscine series, while the ribs of Ganoids and Teleostei are not homologous with 
those of Elasmobranchii; or the ribs are homologous in all the piscine groups, and the 
haemal arches in the tail are differently formed in the different types. Gotte has 
brought forward a great body of evidence in favour of the first view; while 
Gegenbauk,+ may be regarded as more especially the champion of the second view. 
One of us held in a recent publication J that the question was not yet settled, 
though the view that the ribs are homologous throughout the series was provisionally 
accepted. 
It is admitted by both Gegenbaur and Gotte tha/t in Lepidosteus the ribs, in the 
transition from the trunk to the tail, bend inwards, and finally unite in the region of 
the tail to form the ventral parts of the haemal arches, and our researches have 
abundantly confirmed this conclusion. 
Are the haemal arches, the ventral parts of which are thus formed by the coalescence 
of the ribs, homologous with the haemal arches in Elasmobranchii ? The researches 
recorded in the preceding pages appear to us to demonstrate in a conclusive manner 
that they are so. 
The development of the haemal arches in the tail in these two groups is practically 
identical; they are formed in both as simple elongations of the primitive haemal 
processes, which meet below the caudal vein. In the adult there is an apparent 
difference between them, arising from the fact that in Lepidosteus the peripheral parts 
of the haemal processes are only articulated with the basal portions, and not, as in 
Elasmobranchii, continuous with them. This difference does not, however, exist in 
the early larva, since in the larval Lepidosteus the haemal arches of the tail are 
unsegmented cartilaginous arches, as they permanently are in Elasmobranchii. If, 
however, the homology between the haemal arches of the two types should still be 
doubted, the fact that in both types the haemal arches are similarly modified to 
support the fin-rays of the ventral lobe of the cauda] fin, while in neither type are 
they modified to support the anal fin, may be pointed out as a very strong argument 
in confirmation of their homology. 
The demonstration of the homology of the haemal arches of the tail in Lepidosteus 
# “ Beitrage z. vergl. Morph, d. Skeletsystems d. Wirbelthiere. II. Die Wirbelsaule u. ihre Anhange.” 
Archiv. f. Mikr. Anat., vol. xv., 1878, and vol. xvi., 1879. 
f “ U. d. Entwick. d. Wirbelsaule d. Lepidosteus , mit. vergl. Anat. Bemerkungen.” Jenaische Zeitsckrift, 
Bd. iii., 1863. 
X ‘ Comparative Embryology,’ vol. ii., pp. 462, 463. 
MDCCCLXXXII. 3 E 
