623 
DEVELOPMENT OF THE SKULL IN THE LACERTILIA. 
Before passing to the next stage I must mention that, according to Mr. Balfour, 
the walls of the head-cavities become composed, as the cavity closes, of columnar cells, 
and that these become transformed into muscles; and that it is “ almost certain that 
we must regard them as equivalent to the muscle-plates of the body, which originally 
contain, equally with those of the head, sections of the body-cavity. If this determination 
be correct, there can be no doubt that they ought to serve as valuable guides to the 
number of segments which have coalesced to form the head.”—(p. 208.)" 
Fourth Stage. Embryos of Lacerta agilis, \ inch long; head, fh of an inch. 
This is a very instructive stage, coming between the last and the fifth, in which 
the metamorphosis is rapidly completing itself. 
I give of these a direct side view of the most mature (Plate 38, fig. 3); an under 
view of a more immature embryo, tilted back (Plate 37, fig. 8); an under view, tilted 
forward, of the riper embryo (Plate 38, fig. 4); and a dissection of the palate of this 
last seen in a directly lower aspect (Plate 39, fig. 1). 
The mesocephalic flexure still exists; the vesicles of the brain are very bulbous; 
the nasal sacs {ol.) are distinct crescents, widely spreading under the fore face, almost 
repeating the form of the budding hemispheres (C ]ff ) under which they grow. 
The orbit is now well formed ; there is the very solid palatine fold beneath ( mx.p.), 
notched off from the nasal sac {ol.); and over and around the eyeball (<?.) the super¬ 
orbital thickening ( s.ob.). 
The rudimentary mandibles (mn.) are still very arrested, leaving the gape wide 
open (Plate 38, figs. 3 and 4); but beneath (fig. 4, mn., hy.), the two first post¬ 
orals are united at the ventral fine: the branchials are separated by the peri¬ 
cardium ( pcd.). 
The clefts are still open, and especially the first, or “ tympano-eustachian ” (Plate 38, 
fig. 3 , cl?). It is a large lozenge-shaped, gaping space, through which the cavity of 
the fauces and the wall of the ear-sac {ait.) can be seen. 
The mandibular operculum partly overlies this hole, and it is, externally, a con¬ 
tinuation of the skin which is so thick behind the eye and above the angle of the 
mouth, where the ingrowing mandible can be seen to be hinged. 
This toothless, wrinkled, bald head, bears a striking similitude to what the highest 
Vertebrate head degenerates into, when the life-processes are working feebly, and in a 
retrograde manner. 
The opercular fold of the oral cleft overlaps the hinge of the mandible; that of the 
mandible, above, the fore part of the great tympanic cleft; and the fold on the 
'* The kind of work which the Embryologist does differs in toto from the “ weaving of fine cobwebs ” 
out of the “ Philosophical Anatomist’s ” own consciousness, and then entitling those films of the fancy by 
such high-sounding words as “Exemplars” and “Archetypes.” 
MDCCCLXXIX. 4 L 
