THE GILL-ARCHES OF MAN, 307 
extremities are as yet short, broad lumps, at the ends of 
which the foundations of the five toes are placed, connected 
as yet by amembrane. At a still earlier stage (Fig. A—D) 
the five toes are not marked out at all, and it is quite im- 
possible to distinguish even the fore and hinder extremities 
from one another. The latter, as well as the former, are 
nothing but simple roundish processes, which have grown 
out of the side of the trunk. At the very early stage 
represented in Fig. 7 they are completely wanting, and the 
whole embryo is a simple trunk without a trace of limbs. 
I wish especially to draw attention in Plates II and 
IIL, which represents embryos in early stages of develop- 
ment (Fig. A—D)—and in which we are not able to recog- 
nize a trace of the full-grown animal—to an exceedingly 
important formation, which originally is common to all 
vertebrate animals, but which at a later period is trans- 
formed into the most different organs. Every one surely 
knows the gill-arches of fish, those arched bones which 
lie behind one another, to the number of three or four, 
on each side of the neck, and which support the gills, 
the respiratory organs of the fish (double rows of red leaves, 
which are popularly called “ fishes’ ears.”) Now, these gill- 
arches originally exist exactly the same in man (D), in dogs 
(C), in fowls (B), and in tortoises (A), as well as in all other 
vertebrate animals. (In Fig. A—D the three gill-arches of 
the right side of the neck are marked k, k, k;). Now, it 
is only in fishes that these remain in their original form, and 
develop into respiratory organs, In the other vertebrate 
animals they are partly employed in the formation of the 
face (especially the jaw apparatus), and partly in the forma- 
tion of the organ of hearing. 
