MAMMALIA, 475 
radioulna or tibiofibula. The proximal ends of the radius and ulna 
both take part in the formation of the facet or sigmoid notch in which 
the condyle of the humerus moves, and the ulna is always produced 
backwards as an olecranon process for the insertion of the triceps muscle. 
This olecranon part of the ulna remains in cases where the ulna 
atrophies, hence the radioulna or ulna has its facet deep and not 
quite at the proximal end of the bone. Distally both radius and ulna 
have shallow facets for articulation with the proximal carpals. 
Tibia and Fibula.—The proximal ends of the tibia and fibula both 
usually take part in the formation of the shallow facet of the knee-joint 
upon which moves the distal end of the femur. 
Their proximal end therefore has a shallow facet which is a¢ the 
extreme end, Notice that of the two girdle-joints ; that of the fore-limb 
or the glenoid joint is less deep than that of the hind-limb or the 
acetabulum, But in the case of the limb-joint that of the fore-limb or 
elbow-joint is much deeper than that of the knee-joint. By keeping 
these points in mind there should be no difficulty in recognising a 
radioulna from a tibiofibula or an ulna from a tibia. 
Manus and Pes,—The wrist-bones or carpus and the ankle-bones 
or tarsus require special study to be distinguished one by one, but the 
astragalus and calcaneum are always fairly characteristic, the former 
bearing a well-developed sigmoid head for the tibia and the latter being 
produced into the heel in which is inserted the tendon of Achilles. 
The metacarpals and metatarsals are remarkably developed in the 
Ongulata. In correlation with a reduction in the number of the toes, 
those remaining are correspondingly increased in size, forming the 
cannon bones of the horse and ox. These have the appearance of the 
true long-bones of the limbs, but they may at once be recognised by 
having a hollow facet at one end (proximal) and a bilateral condyle at 
the other end (distal). 
DEVELOPMENT.—In studying mammalian development 
we have to keep in mind that the larval and lecithal nutri- 
tions have been given up and that there is a succession of 
three forms of nutrition—the albuminal, the hemal and 
the lacteal. The Prototheria are oviparous, 7.e., the young 
are discharged from the body as eggs surrounded by a shell, 
and further development takes place outside the body of the 
parent; but the great majority of the Mammalia are vivi- 
parous, z.¢, the young are retained during early stages in 
a special part of the oviduct, called the uterus, and are 
“born” later. 
MATURATION AND PRODUCTION oF THE Ovum.—The 
eggs arise in the ovaries which are paired. The outer epi- 
thelial layer of the ovary is the germinal epithelium, and 
from it the eggs sink into the underlying connective tissue 
