540 
ME. W. K. PAEKEE ON THE STEUCTUEE AND DEVELOPMENT 
sigmoid in shape, lessen rapidly in the series, and are so delicate that in some parts 
they are only composed of a single row of cartilage-cells, arranged crosswise. 
The slight shading at the apices of the first three branchial arches shows where the 
fingered gills arise. 
The nasal involution (fig. 1, ol.) is becoming differentiated into layers ; the eyeball ( e .) 
is almost perfect, and the ear-ball ( au .) shows through its membranous walls seven or 
eight crystals, the otoconial particles ( ot .). Behind the ear-sac and ganglion is a gland, 
evidently the 64 thymus ” ( tm.g .). 
Third Stage. Axolotl “ fry ” 5 to lines long. 
The process of development takes place rapidly in larvae that have become free, 
although they do not increase very fast in size — not above a line in the first week after 
hatching. 
Nevertheless they soon take on the form of the adult perennibranchiate individuals, 
although the head at first is relatively very large. 
The specimen whose skull I now have to describe was only the twenty-fourth of an 
inch longer than the last; yet its cranial bend was much lessened, and the short lower 
jaws had grown, so as to give the “underhung” form to the face seen in larval Pipce 
and Pactylethrce { Plate 22. fig. 3, mJc.). 
The relationship of the Urodele to the Batrachian is shown in many ways; but the 
time at which any particular morphological stage is attained in either is very different ; 
processes that in one case take weeks, and even months, in the other may be gone 
through in as many days. 
But this is the case to a lesser degree within the margin of each of these groups, 
so that particular types break the fall from the one to the other. 
Here is an instance of the slowness of growth and change of one part and the quick- 
ness in another: — In larvae of the Common Toad (Phil. Trans. 1876) the trabeculae are 
as much developed in the first stage as they are in the Axolotl in individuals two thirds 
of an inch in length, and intermediate between my third and fourth stages (Plate 23. 
figs. 1, 2, and Plate 22. figs. 4, 5). 
But the mandible, its pier, and the relation to it of the hyoid cornu — these are as far 
forwards in the beginning of my third stage in the Axolotl (Plate 22. fig. 3, mk., chy.) as 
they were in Toads whose metamorphosis was almost complete, the tail having disap- 
peared (Phil. Trans. 1876, part 2, plate 55. fig. 6). 
A sphenoidal neural crest has now grown up from the middle of each trabecula ; but 
the hinder end is flat, embracing the notochord, and the fore end rounded (Plate 23. 
figs. 1, 2, nc., tr.). 
The mandibular pier inclines very little forward at present. After growing forwards 
in succeeding stages, it retires somewhat, but in this type never recovers the vertical 
position. In tadpoles of the Common Frog and Toad at this stage this bar is almost 
horizontal, but it was vertical in the first stage (“ Frog’s Skull,” plate 3. figs. 2 & 3). 
