112 PROCEEDINGS OP THE AMERICAN ACADEMY 



he insisted upon their homology with the gill slits, especially of sharks, 

 and therefore also designated them " Kieraeuspalten " (branchial clefts). 

 An embryonic horse two thirds of an inch long, which had lain some 

 time in spirit, was taken for comparison. Although the external wall 

 of the throat was smooth, the inner surface showed on either side four 

 moderately deep furrows, having the same general form and position 

 as the clefts in the case of the pig. Rathke thereupon came to the 

 conclusion, that the embryos of mammals generally are provided with 

 gills, and that the clefts are gradually closed by a fusion of their walls, 

 which progresses from without inwards. This was, in substance, the 

 important investigation which Von Baer afterwards alluded to as 

 " Kathke's brilliant discovery of gill clefts." 



Iluschke ('2G, col. G20) communicated to the assembly of German 

 naturalists in September, 1825, his studies on the development of the 

 frog and other amphibia. In the frog the gill apparatus becomes the 

 middle ear, in that the inner opening of the first cleft becomes con- 

 verted into the Eustachian tube. Iluschke ventured to go farther, and 

 to predict that in birds and mammals the first of the clefts recently 

 discovered by Rathke becomes converted into the external meatus. 



In the following year Iluschke ('27, col. 401) extended his observa- 

 tions to the chick, where he found that between the two arches (man- 

 dibular and hyoid) there was a large hole which opened only a little 

 farther back than the first gill cleft, and also at first led into the mouth 

 cavity. It was no longer a gill opening, but external meatus, whereby 

 it was in his opinion still better (than in frogs) established that this 

 opening (i. e. as well as the Eustachian tube) possesses in the bran- 

 chial fissure its first outline. 



Rathke ('28, col, 80-85) soon after reported that he had not seen 

 this hole, and he expresses a doubt as to its being the external meatus, 

 even if it does exist. 



Iluschke subsequently- ('28, col. 162) reaflirmed his interpretation 

 of this opening in an article accompanied with illustrations. From the 

 fourth to the fifth day a fine hair could be passed without resistance 

 through this opening and the Eustachian tube into the throat and 

 mouth, from which he inferred that the tympanic membrane was not 

 yet formed. Von Baer ('28, p. 77), describing the chick of the fourth 

 day of incubation, simply says that he observed a deep depression in 

 the bottom of the pharynx pointing toward the ear, and that it was 

 probably the beginning of the Eustachian tube. Early in the fifth 

 day the first gill slit becomes unrecognizable (p. 83). The ear is indi- 

 cated by a round elevated ridge, but the depression which it surrounds 



