1890.] The Homologies of the Fins of Fishes. 417 
ble however that Thacher’s opinion is correct, but not for the rea- 
sons which he gives. I have oberved in a memoir On the 
Mechanical Origin of the Hind Parts of the Mammalia,” that 
“the mechanical cause of the origin of neurals pines may be 
traced to the strains upon the vertebral axis caused by a primary 
dorsal fold, or fin.” ® On this view the actinophores and the 
neural spines were simultaneously developed in lines of strain 
which naturally extend to the point of resistance nearest to the 
moving fin-fold, viz.: the apex of the neurapophysis. The de- 
velopment of the segments neural spine, axonost, and baseost 
was then simultaneous, and the one segment was not derived 
from the other. Thus the views of both Gegenbaur and Thacher 
are partially justified. 

Fic. 8.—Undina penicillata, one-third natural size, showing rhipidopterygian and 
f Undi tidens ; from Zitte 

actinistious types of fins; 7, jugular plates; 4, scales of U i: 
The development of the appendicular skeleton is now shown to 
have proceeded from without inwards, and thus the homologies of 
the parts of the actinophores of all the fins must be interpreted 
from the same point of view, z.¢., from without inwards. 
30 American Journal of Morphology, 1889, p. 210. 
