The Morphology of the Tadpole of Xenopus laevis. 
251 
from the end of the opercular cavity ; its position is against the latter 
view. 
Lachrymal Canal. — In more advanced tadpoles there is on each side 
a canal which, at one end, opens into the nasal sac just beside the external 
opening of the latter ; from here it runs backwards and outwards, opening 
to the exterior by a double pore just above the articulation of Meckel's 
cartilage, behind the attachment of the tentacular bar. Its position, 
running from the nose to a region in front of the eye, and its double 
aperture suggests its homology with the lachrymal canal of reptiles, birds, 
and mammals ; but it cannot be described as a duct running between the 
trabeculae and the palato-pterygo-quadrate bar as the lachrymal duct should, 
according to Foster and Balfour.''' Perhaps it is to be compared to the 
second nasal opening of Ganoids and Teleosteans. 
Visceral Skeleton. — I have already mentioned the skeletal rods of the 
tentacles and expressed the opinion that they are homologous with labial 
cartilages (TO of Figs. 6 and 7). 
First Bar. — The palatal portion is stout and is divided into two portions 
by a small foramen. The two portions may perhaps be the prepalatine 
cartilage (PPa) and the palatine (Pa). The articulation of Meckel's 
cartilage is far forward, but the pedicel (Ped) has also a large outward 
projection (Q), and I am of opinion that the latter (Q), and not the larval 
articulation, is the quadrate cartilage of the adult. Eeasons for such an 
opinion are : — 
1. The statement, common to textbooks, that during metamorphosis 
the quadrate articulation shifts backwards cannot be taken literally since 
then the palatines, which are necessarily the regions of growth for such 
a shift, would have to be directed obliquely backwards in the adult ; 
instead, the palatines always point directly outwards. 
2. The position of the pterygoid portion (P) would be shifted rela- 
tively to the palatine and quadrate unless the articulation gradually 
shifted back along it ; but the change is rapid and could be much better 
explained by saying that the old articulation is retained ; that during the 
late larval life Meckel's cartilage grows back past the articulation until it 
forms another articulation with (Q) on the pedicel; that during meta- 
morphosis the larval articulation (with the palatine) is lost and the larger 
gape thus formed. 
3. A figure by Milne Marshal, reproduced in Selenka (" Zoologisches 
Taschenbuch ") for Eana, indicates as the quadrate a knob of cartilage 
behind the articulation of Meckel's cartilage. 
4. The cartilage (Q) is connected distally with a plate of imperfectly 
chondrified connective tissue which may be regarded as the precursor 
of the cartilaginous ring around the middle ear of the adult. 
, * Foster and Balfour, " Elements of Embryology," p. 248. 
