THE SKELETON 81 
month of foetal life, giving rise to a prominence which can be 
recognised in the adult. This prominence is present also in the 
Chimpanzee, the Gorilla, and Hylobates; and as the centrale is 
most probably distinct in the embryo of these, it may well be 
that in them, as in Man, its independent existence has not long 
been suppressed. Further confirmation of this is afforded by the 
fact that it is still an independent bone in 0:4 per cent of even 
adult human beings, and that, normally, it retains its independence 
in the Orang and in most Monkeys. 
In many Mammals (especially Marsupials, Rodents, and Insectivores) 
cartilaginous or bony skeletal elements occur on the outer and inner borders 
of both fore- and hind-limbs, which not only bear a superficial resemblance 
to the digital skeleton, but may in some cases be clad, like the true digits, in 
either a claw ora callous horny integument. Similar structures occur in 
many of the lower Vertebrates (Reptiles and Amphibians). These organs 
were formerly considered by both von Bardeleben and myself to be vestiges 
of now vanished digits, and were named by us ‘‘ preepollex,” ‘ preehallux,” 
and ‘ postminimus.” 
I have, however, entirely changed my opinion as to the supposed atavistic 
nature of these structures, and now agree with others that these “super- 
numerary rays,” whether they occur in the lower or the higher Vertebrata, 
are to be regarded rather as progressive structures of convergent and second- 
arily adaptive significance. Baur has contended, before all others, that the 
facts of paleontology favour the view that the terrestrial Vertebrata never 
possessed more than five rays in the skeleton of either fore- or hind-limb ; ! 
and my own recent investigations into the development of the limb-skeleton 
entirely confirm this conclusion. 
From this point of view, the condition of ‘ hyperdactyly,” which not 
infrequently appears in Man and is often inherited for many generations, 
loses its supposed atavistic significance. 
THE SKELETON OF THE HIND-LIMB 
The human femur usually bears at its head two processes for 
muscular attachment, known as the trochanters, inasmuch as 
they give insertion to the rotator muscles of the limb. Special 
interest centres in the not infrequent presence of a third trochanter 
(ic’’., Fig. 58), a development of the roughened area (tuberositas 
glutealis) which occurs on the external border of the bone 
1 [It is an interesting corollary to this, that the only fossilised limb in which any- 
thing comparable to a sixth digit has been found, is a fore-limb which, if not actually 
Mammalian, is that of a Reptile with Mammalian characters ( Zheriodesmus, from the 
Mesozoic beds of South Africa, cf. von Bardeleben, Proc. Zool. Soc., Lond., 1889, p. 
259 ; and Seeley, Proc. Roy. Soc., Lond., vol. lv. p. 227). Nor must it be forgotten 
that the ‘‘ prehallux” in its most highly differentiated and digit-like form (Frogs 
and Toads) is cartilaginous, 7.e. so constituted that it would not be preserved in the 
fossil state. ] 
G 
