1892] The Head of an Embryo Amphiuma. 675 
laginous lower arch and in the fibrous connective tissue on 
either side of the notochord is a deposit of bone of such a 
character as to suggest the existence here of an earlier verte- 
bral centre which has disappeared. 
VISCERAL CLerts.—My specimens are too old to throw any 
light upon the mooted question of an obsolete visceral seg- 
ment between mandible and hyoid, but in the region behind 
the last branchial cleft of the ordinary Amphibian some inter- 
esting facts are seen. A reconstruction of the floor of the throat 
after the method of Born shows the following clefts distinctly : 
—a, the hyomandibular or spiracular cleft, which like b and c, 
the first and second branchials, is not open to the exterior; d, 
the third branchial cleft which is still functional, opening to 
the outer world as already described in referring to the exter- 
nal appearance. Behind this last cleft comes the fourth carti- 
laginous gill arch; and still behind this and between it and 
the trachea are two other pits, clearly serially homologous with 
the others, and hence to be regarded as the representatives of 
the two posterior clefts of the typical elasmobranchs and gan- 
oids. Of these the anterior (fourth branchial) has already 
been recognized as occurring in the Amphibian ontogeny ; it 
is the “ Suprapericardialkérper ” of authors, which Maurer has 
shown to be the fourth gill cleft. The posterior, the fifth gill 
cleft has not before been recognized in the Batrachia. These 
posterior clefts bear such relationships to the trachea as to lend 
countenance to that view which would derive lungs and trachea 
from modified gill slits. Should this view ever be substan- 
tiated, it may be that the laryngeal cartilages will be shown to 
be the modified gill-bars of this region. Amphiuma, however, 
throws not the slightest light directly upon the phylogeny of 
these structures. 
In this connection I may state that in the early Siredon 
stage of Amblystoma jeffersonianum the posterior (fourth) 
branchial cartilage is bifid at its upper and posterior extrem- 
ity? in such a manner as to suggest that there was formerly 
here an additional arch, the traces of which are disappearing 
in the same way in which the posterior gill of Ichthyophis is 
2This, of course, bears no relationship to the bifid ceratohyals of the ganoids. 
