MAMMALS. 221 



second. The rneso-sternum becomes gradually very broad and flat, and its edges arc produced into 

 pedicillate, oblique cups, for the sternal ribs (s. r.) : this pedicillation of the edges of the Sternum 

 for the ribs is exactly what is seen in the Oviparous embryo. The xiphoid (x.) first narrows, and 

 then expands towards the end, which is at present bilobate. Except in the first pair, a " cup 

 and ball" synovial joint is found between the vertebral and sternal ribs ; the latter are totally 

 unossified at this stage ; an cctosteal sheath is creeping down the former. In an Ox-embryo six 

 inches six lines in length, ossification of the Sternum has commenced, for now the cartilage has 

 acquired a considerable density. Sections through the fifth meso-sternal region (figs. 5 and 5, 

 m. st. 5, four diameters) show an osseous centre (pleurosteon), which towards its end (fig. 5) only 

 reaches the upper surface of the cartilage, but in the middle (fig. 50) has grown through the sternal 

 plate. This, however, is not an ectosteal patch, for a section through the second meso-sternal 

 region shows an endosteal patch lying in the very centre of the cartilage (fig. 4, m. st. 2). The 

 sternal ribs are convex at both ends (figs. 3 and 4 ; in fig. 3 the ribs are in section, and the 

 Sternum is shown from the cleft ; whilst in fig. 4 the Sternum is in section and the ribs are entire : 

 these views display the synovial cavities, and the height of the anterior part of the Sternum). The 

 Sternum of an adult Ruminant (fig. 6, upper view, Tragulus javanicus, nat. size) is well ossified, 

 and secondary centres (ep.) appear at the costal hinges. In this type there is some fibrous seg- 

 mentation at the second and third interspace : a slight degree of carination is seen above ; but 

 below the sternals are flat, and, indeed, somewhat concave. Here (fig. 6, p. st.) there is a 

 cartilaginous cervical rostrum to the manubrium ; the seventh and eighth sternal ribs are attached 

 to one piece, but the eighth belongs to the spatulate xiphoid (x), which is soft behind. The 

 breadth of the hinder part of the raeso-sternurn is remarkably seen in the Camel, and the bones 

 are extremely thick as well as broad. Contrary to what is seen in the Vertebrate Sternum gene- 

 rally, that of the Ruminant is widest below ; this is in conformity with the posture assumed by 

 animals that chew the cud, as they lie "bedward ruminating." 



In the Ass-foal (two weeks old) the Sternum is composed of one piece, thick behind, boat- 

 shaped, and compressed in front (Plate XXIX, fig. 13, nat. size) ; its regions are marked out by 

 the articular fossae, and by elliptical patches of bone : these " pleurostea" are enclosed in the 

 cartilage (see figs. 14 and 15, sections taken horizontally, and transversely vertical, two diame- 

 ters). The "prow" of the prse-sternum (fig. 13, p. st.),has no osseous centre, and large inter- 

 spaces of cartilage separate the bony nuclei. I can find no trace of a pras-mesosternal cleft. In 

 the adult Horse (fig. 16, one sixth nat. size) the ossification is not, relatively, very much advanced; 

 but the sternal ribs have a bony core, and there is an endosteal mass above the joint-cavity in the 

 second vertebral rib, which feebly indicates the " intermediate rib." In an Ass accounted forty, 

 and in a Horse known to be above thirty years of age, I found, in my early dissections, a 

 very imperfect ossification of the Sternum, headlands of cartilage still remaining, and the bone 

 itself light and spongy. 



In the Hippopotamus (fig. 17, young, and fig. 18, old ; both one eighth nat. size) the Sternum 

 is similar to that of the Horse, but the " prow" is longer and more dilated (vertically) : in this 

 rounded part there appears, ultimately, a " pro-osteon" (pr. o.). I cannot find true segmenta- 

 tion of the cartilage between the pre- and meso-sternum ; nor, indeed, can I in the Indian Tapir 

 (fig. 19, adult, one fifth nat. size). Here the "pro osteon" (pr. o.) is a more definite segment, 

 and almost synchronous with the first " pleurosteon" (pi. o. 1). The form of the " prow" in the 

 Tapir is intermediate between that of the Horse and River-horse. In the hinder part of the tneso- 



