NOTE ON THE DEVELOPMENT OP AMPHIBIANS. 315 
at n. II, where they join the forwai’d or doi’sal edge of the 
anterior band of fibres (A. F.). 
Appendages and Skeleton of the Head. 
The gill-clefts develope in Amblystoma after the usual 
manner from before backward. The first or hyoid cleft (I) 
does not break through, but forms like the others a laterally 
extended hypoblastic pouch (figs. 15 f, and 26). In the case 
of the hyoid this pouch extends in a venti’al and median direc- 
tion, forming a groove which meets a similar groove from the 
opposite side. The median portion of this groove is shown in 
the longitudinal vertical section of fig. 18, Th. From com- 
parisons with the work of other writers I suppose this part 
marked Th. to be the rudiment of the thyroid gland, though in 
this case I have traced the development no farther. Whether 
this relation of the thyroid rudiment to the hyoid clefts can 
be considered as an argument for the phylogenetic origin of 
the thyroid gland from the ventral coalition of the hyoid clefts, 
is, I think, doubtful. The ventral groove may be the result of 
the early development of the tongue-rudiment. In the Lizard 
the hyoid clefts are widely opeix to the outside, and the thyroid 
rudiment appears between the transverse areas of the hyoid 
and first branchial clefts. The thyroid rudiment in the Lizard 
has no appai’ent connection with the hyoid clefts. 
In the stage represented in fig. 15 f, the hyoid ( I) and the 
first two bi’anchial cleft-rudiments (II, III ) have appeared ; in 
the stage of fig. 26 five in all have appeared (I — V), but none 
of them have as yet broken through. These stages show the 
development of the head-cavities or mesoblastic somites of 
the head. The anterior somite is the first to develope, and 
appeal’s just behind the eye. The other somites are separated 
off from this first one by the successive development of the 
hyoid and branchial clefts. These somites of the head do not 
attain a characteristic development as cavities as is the case 
with Elasmobranchs and the Lizard. Nevertheless there is 
here a tendency in that direction, and sometimes a slight cavity 
