OF ARTS AND SCIENCES. 113 



remains very inconspicuous during the fifth day. Although the ear 

 appears to have at this time an opening inward by means of the Eusta- 

 chian tube, the external orifice is ordinarily formed on the following 

 (sixth) day, so that it makes its appearance when the gill slits are 

 closed. Still it was sometimes seen when one or the other of the clefts 

 remained (p. 87). In the same year he (Von Baerj '28% p. 147) criti- 

 cised Huschke's view in the following words : " The circumstance that 

 the most anterior gill cleft is early reduced in length, and that its upper 

 persists longer than its lower part, appears to have induced Huschke 

 to regard it as the opening of the ear. So much is certain: the outer 

 orifice of the ear can have nothing in common with the gill apparatus, 

 since the ear belongs to the superior half of the animal (i.e. the 

 Ruckenplatte),and not to the inferior half (Bauchplatte), to which the 

 gills belong." Von Baer admits, however, that the Eustachian tube is 

 a prolongation of the superior half of the body into the inferior part. 



In answer to this criticism of his theory of the transformation of 

 the first cleft, Huschke ('32, pp. 40, 41) declared that Von Baer was 

 in error in saying that the entire ear belongs to the superior half of 

 the body; only the labyrinth is formed from it, the accessory parts 

 being formed by the metamorphosis of the superior part of the first 

 cleft. Huschke supported his position by renewed investigations, and 

 by arguments based upon malformations observed in certain cyclopean 

 monsters. The first pair of clefts, at first widely open, becomes, he 

 says, progressively closed from the ventral median line backwards and 

 upwards, and thus become widely separated, until at last only the outer 

 or upper angle of each fissure is left as a hole, — the meatus auditoriiis 

 externus, — which leads directly into tlie throat by means of the Eusta- 

 chian tube without any tympanic enlargement. 



Valentin ('35, p. 211) concludes from his own observations that it 

 is not to be doubted that the Eustachian tube is the remnant of the 

 inner end of the first visceral cleft, but is uncertain whether tlie tym- 

 panic cavity and the external meatus are " formed out of the whole 

 external part of the cleft." His objection is based on the fi^ct, that the 

 first indication of the external opening of the ear does not lie in a line 

 with tlie fused lips of the cleft, but '' apparently above it in the sub- 

 stance of the posterior boundary of the first visceral arch." 



Reichert's ('37, pp. 149-155) studies were made principally on the 

 pig. He was the first to follow the successive changes of the hyo- 

 raandibular cleft through a numerous series of embryos.* In many 



* It was l>eichert, also, who introduced the terms " Visceralbogen " and 

 Visceralspalten." 



VOL. XIS. (n. S. XI.) 8 



