THE GILL- ARCHES OF MAN. 307 



extremities are as yet short, Ifroad lumps, at the ends of 

 which the foundations of the five toes are placed, connected 

 as yet by a membrane. 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 

 III., 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 othei 

 vertebrate animals. (In Fig. A — D the three giU-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 or^an of hearin^j. 



