174 On ike Double Type of the Reapiratory Organs. 



led to make'an application of the principles of his theory of herma- 

 phrodism to the respiratory organs of the vertebrate animals ; af- 

 firming that the type of the respiratory organs is double ; that is 

 to say, that there are originally the elements of both lungs and gills; 

 but that, in those animals which breathe by lungs, the elements of 

 the gills are not developed ; and in those animals which breathe by 

 gills the lungs are not developed. 



According to this idea, gills, although analogous to, are not iden- 

 tical with lungs, seeing that they may exist together in the same 

 animal. 



The theory of the double type of the respiratory organs is ren- 

 dered complete by the observations of Professor Rathke,* who has 

 detected rudimentary gills in chicks, in embryos of several of the 

 mammalia, and in the embryo of the lacerta agilis. 



According to him, the following peculiarities are observed on the 

 outside of the lower and lateral parts of the neck of the chick of 

 the third, but particularly of the fourth day of incubation, (fig. 1 

 and 7 :) immediately behind the opening of the mouth are two 

 broad and thick lobes (c, fig. 7,) belonging to the two lateral halves 

 of the chick, and formed of the same gelatinous substance as the 

 rest of the body. 



These lobes are incorporated together below, on the mesial line 

 of the body, but in such a way that their point of union is indicat- 

 ed by a broad and superficial groove. 



There is, moreover, at the lower aspect of each lobe, a transverse 

 shallow groove, which divides the lobe into two parts. Its ante- 

 rior part, becoming developed, forms the lower jaw. The poste- 

 rior makes, with its posterior margin, which is rather sharp, a 

 slight projection downwards, and covers by its posterior region the 

 foremost fissure, which is the largest of the neck, nearly in the 

 same way as the operculum covers the giUs in most fishes.t Be- 

 hind this part, the cervical region becomes a little narrower below ; 

 now it is on this contraction, (that is to say, at some distance be- 

 hind the kind of operculum just mentioned,) that the orifices of the 

 two other fissures of the neck are found. The upper and smallest 

 I)ortion of the cavity which lodges the heart, (e, fig. 7,) begins im- 

 mediately behind the two lobes of the cervical region. At the third 

 and fourth days of incubation the heart is as yet only formed of owe 

 ventricle (e) and of one auricle (f;) and it gives off the aorta, (rf). 



The anterior part of the arterial system of birds has, at the be- 

 ginning, the same disposition as that of salamanders at the first 

 period of development ; that is to say, birds have vessels which pre- 

 sent a striking resemblance to the branchial vessels of salamanders, 

 during the first period of their development. 



• Act. Cur. Nat. Bonn. 1829. Tom. XIV. Part I. 



f According to this, the bones of the operculum are to be looked for in the 

 lower jaw, and not, as M. St. Hilaire thinks, in the small bones of the ear. This 

 is the more likely, as there is a fish in which their is an appearance of rudimen- 

 tary ossicula auditus. 



