OF THE SKULL IN THE UKODELOUS AMPHIBIA. 
531 
These things relate to extant and familiar forms ; but types like the Urodela, that 
possess external branchiae (all for a time, and many persistently) and that never have 
internal branchiae, must, it appears to me, be related to groups of extinct forms of low 
vermiform kinds of Fish, far below any known brain-bearing fishes now existent. 
Indeed to me the interest attached to every stage, stage by stage, in these metamor- 
phosing Ichthyopsida has been greatly enhanced by the supposition that each of these 
grades shows me a pattern according to which family after family of the finny tribes 
was developed — modified, undoubtedly, in these forms by exoskeletal structures that 
gave the members of each group a perfect fitness for their own conditions of life. 
I feel that I need only suggest this view of the matter for it to be accepted by the 
thinking zoologist and anatomist ; the contemplation of such forms as I figured in my 
last paper, namely, the larvse of Dactylethra , and the larvae here figured of Siredon, 
unconsciously set the imagination to work in an attempt to pass the great gulf at present 
fixed between the brainless Amphioxus and the lowest brain-bearing Fishes extant, such 
as the Hag-fish and the Lamprey. 
That, however, is not the primary purpose of the work of which the present paper 
forms a part; the goal aimed at is the interpretation of the skull as part of the Verte- 
brate skeleton. This, in detail, means to ascertain what relation the cranium bears to 
the vertebral column, the visceral arches to the ribs, and the subcutaneous cartilages to 
those superadded arches of the body that sustain the limbs. 
The morphological space traversed by a Salamander or Newt in passing from the first 
fixation of the cartilaginous beams and bars that form its first chondroskeleton, and 
especially its first chondrocranium, to its adult state is immense ; the first grade shows 
the framework of a fish lower by far than any save the Lancelet, and the last lands us 
among forms that lie on the border of the true Reptilia. 
Here, amongst these types, must the morphologist search for the alphabet, and, as the 
mind learns to arrange the facts, synthetically construct the grammar, of his science. 
To a great extent the writer has studied the relations of both the nervous and mus- 
cular systems as correlated with the skeletal ; here, however, he needs much friendly 
help ; that help has already been given in some degree, but most of the work has yet 
to be done. 
On the Skull of the Axolotl. — First Stage. Unhatched Embryos, one fourth 
of an inch long. 
The larva of Siredon at this stage is entirely (or almost entirely) composed of cells ; 
but its development is, embryologically, far advanced, for in the head, at least, the 
greater number of the organs can be made out. 
Indeed in the specimen figured, which has been preserved for three or four years in 
weak chromic-acid solution, very much that is highly instructive could be seen from the 
exterior; views of these are here given from the side, from above, and from below 
(Plate 21. figs. 1-3). 
4f 2 
