152 
ME. W. K. PAEIvEE ON THE STEUCTHEE AND 
horizontal sections are from embryos somewhat in advance of those which are illustrated 
in figs. 7-10. 
Fourth Stage. — Tadpoles 1 inch long. 
After the digestive canal is complete the growth of the larval Frog is very rapid. 
Th e floating gills become covered with the opercular fold, and at the same time lose 
their individuality amongst copious tufts of the same character which grow both from the 
outer or dermal and inner or mucous surface of the gill-arches. On the right side the 
fold of skin from outside the second poststomal (hyoid) arch coalesces with the skin of 
the thorax; but on the left side (Plate V. fig. 1, op.) there is a vertical oval opening 
for the currents of water. The coalescence of primordially distinct organs had begun in 
earnest in the last stage ; but now, having suffered some two or three weeks to elapse, 
we shall find a most remarkable development of this process of mutual engrafting by 
the direct fusion of closely applied parts, by connective bands in lateral bars, and by 
azygous commissures at the mid line. The sense-capsules are not in the least uniform 
in their relation to the cranio-facial structures ; for the auditory sacs are primarily distinct 
and then coalesce, the cartilaginous “sclerotics” are permanently distinct, whilst the 
nasal labyrinth, at first entirely membranous, has its floors, roofs, and walls entirely de- 
veloped as “ outgrowths” from the first visceral arch (“ trabeculae”) and its prepituitary 
“ commissure.” 
Rightly to understand the skull and face of this perfect Tadpole (the next stages lead 
to the Frog), it will be well to compare the corresponding figures. Beginning with the 
lateral view, let the eye refer back to Plate IV. fig. 7 (third stage), to Plate IV. fig. 1 
(second stage), and to Plate III. fig. 3 (first stage). 
Beginning with the primordial skeleton of the mouth, we see the labials at their 
fullest state of development ; and as they are very important in relation to the various 
ichthyic types, especially the lower, they may now be described. 
When the skin is removed from the side of the Tadpole’s head (Plate V. fig. 1), or 
when the head is completely bisected (fig. 2), then the upper and lower labials ( u.l ., l.l.) 
are seen to form the thickness of the wrinkled dentigerous lips (Ip.) ; between these is 
the nearly vertical, very small, suctorial mouth. 
The upper pair (Plate V. figs. 1-6, u.l.) are relatively large falcate flaps of solid hyalint 
cartilage, the cells of which proliferate rapidly (fig. 6"). They are almost vertically 
placed (figs. 1, 2), are thick and heart-shaped in section (fig. 2), the upper thick edge 
being grooved and embracing the decurved end of the trabecular horn (fig. 2, u.l., tr.). 
They are not the germs of the prsemaxillse, nor the premaxillary axis (endoskeleton)*, 
* In the “ Croonian Lecture ” (p. 33, fig. 9), Professor Huxley named the upper labial “ premaxillary,” 
and the lower “ mandible the error was discovered too late for correction in print, as he now informs me. 
That is almost the only flaw I can detect in his description of the larval amphibian skull : it only extends over 
four or five pages (p. 31 to 35); but that and the small woodcuts illustrating it have been of inestimable value 
to me in working out the present paper ; I should have repeatedly lost myself during my research hut for this 
ever-present guide. 
