46 THE STRUCTUEE OF MAN 



successive pieces. Its early condition is now most nearly retained 

 for Mammals among the Edentata [i.e. in the Pangolin (Mams)], 

 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. 1 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. 



St. 



FIG. 28. SHOULDER GIRDLE OF ORNITHORHTXCHUS. 



m.s., manubrium sterni ; .,<?.,<?., first, second, third ribs ; st., sternebra ; sc., scapula : 

 m.c., metacoracoid ; 2 e.c., epicoracoid ; d, clavicle ; es'. and es"., interclavicle (episternum). 



The origin of the Mammalian 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 (ed., 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 Eeptiles.] 



In Monotremes (Fig. 28) the episternal apparatus (es f . 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 i.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.] 



