On the Double Type of the Respiratory Organs. 175 



At the fifth day of incubation, the two posterior branchial open- 

 ings of each side have already, in certain embryos, so much dimi- 

 nished in extent, that they only form small round holes scarcely 

 visible ; in others, they are completely obliterated (fig. 2 and 8.) 



However, when, in the latter, the pharyngeal cavity is divided 

 and its parietes folded back, there are found on each side, at the 

 places previously occupied by the two last branchial openings, the 

 same number of little apertures, which are the vestiges of the bran- 

 chial cavities, but which are themselves effaced during the follow- 

 ing hours. 



The embryos of the mammalia also present, at an early period, 

 several fissures on the sides of the neck, which, traversing the thick 

 wall of this region, lead into the pharyngeal cavity. The foremost, 

 situated immediately behind the rudiment of the lower jaw, is the 

 largest ; the posterior is the smallest. An embryo of a pig, six lines 

 long, and which was just three weeks old, exhibited not three of 

 these fissures as in birds, but four (figs. 3 and 9, aaaa, 11, cccc.) 



The three first represented small cavities of a lenticular form, 

 flattened from before backwards, the external and internal orifices 

 of which were a little narrowed. The posterior fissure, which had 

 perhaps been previously the largest, formed, on the contrary, a 

 round canal scarcely.perceptible. The anterior cavities were lined 

 by a very delicate membrane, which was a prolongation of that of 

 the broad and long pharyngeal cavity. Professor Rathke supposed 

 he saw, in the interior of these cavities, small parallel projections 

 directed from within outwards, which rose from the surface of the 

 membrane, and resembled the branchial laminae in the respiratory 

 cavities of sharks. However, he does not vouch for the accuracy 

 of this observation, seeing that he might have been deceived by the 

 minuteness of the object, and by a preconceived opinion. 



In the embryo of a horse, six lines long from the vertex to the 

 tail, (fig. 4 and 10.) the openings of the neck were already closed, 

 and in their stead were the lung and the trachea, with the larynx ; 

 but all these parts were still at the first period of their develop- 

 ment. 



As he had expected. Professor Rathke found similar appearances 

 in the embryos of lizards. 



From the analogies just related. Professor Rathke infers that 

 the lamince which exist in the neck of the chick, are rudiments, or 

 one of the lowest of the transitory forms of the gills ; and that 

 birds have gills, which however disappear after the first period 

 of their embryo development. 



Explanation of the Plate, 



Fig, 1. Anterior part of a chick at the fourth day of incubation, 

 magnified two times. 



Fig. 7- Id. merely in outline, a a a, Vesicles which represent 

 the first traces of the encephalon and medulla oblongata, 



\ First trace of the lower jaw. 



