618 
ME. W. K. PAEKEE ON THE STEUCTUEE AND 
ginous, seven pairs of bars, two pairs of labials, and one pair (we do not count the eye- 
balls) of “ paraneural ” elements. 
Most of the cranium, therefore, is “ membrano-cranium ; ” but the foundations of this 
small “ lodge ” are fairly marked out, and are rapidly solidifying by chondrification. 
It is open to me to remark how that this type, even at its early stage, only gives 
evidence of possessing two “ somatomes in its head. However complex what we call 
the skull becomes, it is largely built up upon the foundation of the trabeculae cranii, 
these being the chief elements. The hinder third of the skull is parachordal. 
A figure like this (fig. 1) would have made a proper series in Professor Huxley’s 
valuable plate (“ On Menobranchus ,” plate xxxi. figs. 1 & 2, Siredon and Triton). In all 
the three we see, fairly, in the interauditory or occipital region, two myotomes. Those 
Urodelous types differ from the larval Toad and Frog in the early appearance of the 
bony plates ; they come very much later in the Batrachia. 
Skull of Bufo vulgaris. — Second Stage. Tadpoles 5 lines long. 
During the time in which one of these larvae is one fifth longer, the head has grown 
twice the size, and the development of the parts has gone on very rapidly. The external 
gills are no longer apparent, and the form and appearance is that of a perfect bull-head. 
The chondrocranium at this stage (fig. 4, a half-figure, seen from below) is much more 
perfect than in the last, for the gelatinous stroma that lay over the myotomes on each 
side of the notochord has become hyaline cartilage. I cannot view these parachordal 
tracts (iv.) as the growth backwards of the apices of the trabeculae, but as quickly 
transformed cells, the fore margin of whose territory is lost in the trabecular tract (tr.) 
as soon as the solidification takes place. The notochord ( nc .) has retreated, and has 
its fore margin bounded by a solid belt of cartilage, from which on either side the 
parachordal bands run, binding on the sides of the notochord, and ending behind in 
the occipital condyles. In the Salmon-Try of the second week after hatching (my 
5th stage, op. cit. plate iv. p. 127) the trabeculae and “ investing mass ” are quite distinct, 
the former crossing fingers with the latter in a most evident manner. Here, where the 
chondrification is not synchronous, there need, surely, be no difficulty in the interpre- 
tation of the parts : they are distinct in the Newts. 
The trabeculae, which pass forwards as broad, flattish bands, have coalesced in front, 
forming an internasal plate (see also fig. 3), and now their large flat cornua ( c.tr .) are 
falcate, with a slightly notched convex margin. 
The pituitary space enclosed by them, under the middle brain, is now more regu- 
larly oval ; and the subocular fenestra formed by the trabecula and mandibular pier is 
now widest at its hinder part. A prenarial ligament ( p.n.l ) connects a tooth-like process 
* The TJrodela in an early stage show a subdivision of the notochord into two rudimentary “ centra ; ” this 
is best seen in the larva of Seironota perspicillata. Most of them, thus, have a rudiment, in this way, of a 
basioecipital, which represents two or more centra. 
