46 THE STRUCTURE OF MAN 
successive pieces. Its early condition 1s now most nearly retained 
for Mammals among the Edentata [7.e. in the Pangolin (Manis)], 
and even in the lower Apes extensive remnants of cartilage are 
occasionally present between the bony parts. In most other 
Mammals, the ossific nuclei which appear in the course of develop- 
ment of the sternum are the only indications of its former 
segmentation.’ The fully-developed sternum of the Primates is 
practically a single broad and firm plate, the solidity of which 
compensates for its decrease in length. 
Fic. 28.—SHOULDER GIRDLE OF ORNITHORHYNCHUS. 
m.s., manubrium sterni; cl.,c”.,c®., first, second, third ribs ; s¢., sternebra ; sc., scapula ; 
m.c., metacoracoid ;? e.c., epicoracoid ; cl, clavicle ; es’, and es”., interclavicle (episternum). 
The origin of the Mammalhan interclavicle (so-called epi- 
sternum) is still somewhat undetermined; [but in its position 
beneath (ventrad of) the sternum proper in the young of the 
Mole (es’., Fig. 29), in which its development has been most 
fully worked out, and in its relationships to the clavicles, it agrees 
with the interclavicle of Reptiles. | 
In Monotremes (Fig. 28) the episternal apparatus (es’. es”.) is 
triradiate, and disposed altogether cephalad of the sternum proper. 
1 [Approximation of more than one pair of ribs to the posterior end of the 
sternum is the rule in many of the lower Mammalia ; in the Rabbit, where two pairs 
of ribs always have this relationship, it may or may not happen that a corre- 
sponding extra sternal segment is present in the adult. A careful study of the 
development of that animal’s sternum has shown that this segment disappears by 
absorption where not retained—.e. that a sternal segment may generally, though 
not invariably, be lost during ontogeny. This fact is of considerable interest in 
relation to the belief in a tendency towards abbreviation of the mammalian thorax 
postero-anteriorly (cf. Burne, Proc. Zool. Soc., 1891, p. 159).] 
2 [Until recently known as the ‘‘coracoid” ; cf., however, infra, p. 72. ] 
