232 PROF. W. K. PAEICEE ON THE 



fagoting togetlier of the elements of the shank has not yet taken place, but the 

 purpose hidden in the fundamentally reptilian organism is unfolding itself ; for on the 

 inner side the 1st metatarsal (mt.^) is merely a distal rudiment one-foui-th the length 

 of the others : the 5th reptilian toe had to be strangled in its infancy ; it exists now as 

 a small seed-like grain of true cartilage {mt.^} outside and above the head of the well- 

 developed 4ith {mt}). The phalanges of the 1st toe are as well developed as those of 

 the next three; and as in the Seal (Phoca), what the phalanges want in number — there 

 are only the hco normal joints — they make up in length; and this helps to make the 

 4-lobed amphibian paddle, ordained to become the foot of a creature that not only " on 

 the ground walks firm " but that is capable of perching also, and of sleeping on the 

 perch, strongly pivoted on a single foot. 



The four distal phalanges {dg}'^) are simjile rounded pieces in an amphibian stage ; 

 afterwards they push out the skin and receive from the cuticle a horny sheath, both 

 joint and sheath cixrving into a claw. 



In the 2nd stage (Plate XXIV. fig. 2), besides general growth, we can see a further special 

 differentiation of elements : — the scaphoid or centrale is acquiring its curved and yet 

 trihedral form, and the distal tarsal mass {dt.) has now acquired a right and left flat 

 out"TOwth. over the 2nd and 4th. metatarsals. The arrested 5tli metatarsal is clearly 

 seen in this stage ; and so are the two upper nuclei, the tibiale and fibulare, beneath and 

 in front q/the distal end of the tibia ; the fibula has now become relatively shorter. 



In the 3rd stage we see the result of 48 hours' more growth ; this additional time is 

 sufficient to bring out the promised bird much, more clearly, in this particular member ; 

 the pre-natal transformation is extremely rapid, for the natural stove furnace is very hot. 



The true proximal tarsals, the tibiale and fibulare (Plate XXIV. fig. 3, tb.,fbe.), are not 

 yet fused together to form the shapely condyle ; they are still, and for some time to come, 

 quite distinct from the tibia {t.). The intercalary centrale (figs. 4, 5, c), or " scaphoid," 

 is well formed, and is wedged postero -internally between the tibiale (astragalus) and 

 the inner region of the distal tarsal mass, a part which corresponds to the middle 

 cuneiform bone of our ankle, above the 2nd metatarsal. Clinging to the outer 

 face of the distal tarsal mass, the small remnant of the 5th metatarsal {nit.^) is 

 still visible; the mass itself is now a thick slab of cartilage, thickest in its oldest or 

 middle part. Behind, tliis connate slab is showing its true avian nature ; the plantar 

 tendons are being pro-\dded with grooves and sheaths and pulleys by a downgrowth of 

 the distal tarsal mass (Plate XXIV. fig. 5, d.t.). At present, a larger inner and a lesser 

 outer oblong lobe of cartilage is seen clamping the hind face, above, of the 2nd and 3rd 

 metatarsals {mt^^'^). 



The three main metatarsals are rapidly ossifying and cling closely to each other now ; 

 the 3rd {mt.^) is behind the other two above ; but, below, it reaches out in front of and 

 beyond them, as it carries the largest too. Still, however, this limb has not got beyond 

 the ornithoscelidan stage, for the metatarsals are all distinct, at present. The larger 

 phalanges are now beginning to ossify, the distal joints have become hooked and are 

 now invested with a horny sheath. 



As the limb here figured (Plate XXIV. fig. 4) was from a Chick of the Dorking Powl 



