138 SHOULDER-GIRDLE AND BREAST-BONE. 



Birds, and Batrachia, this bone may be formed in a twofold fashion. In Mammals and Birds it 

 occurs under the form of two very slender longitudinal rods, divided into two lateral halves, 

 and already at an early period consisting of cartilaginous tissue, each of which rods unites itself with 

 the extremities of several ribs of its own side when these project through a small part of the lateral 

 wall of the body. The two halves, therefore, at first, lie at a considerable distance from one another. 

 Gradually, however, these two rods are approximated to one another by the extension and development 

 of the ribs, until, at length, they come into contact throughout their whole length and ultimately 

 coalesce, forming the Sternum. 



As regards the Batrachia, even in those which possess ribs, there are never at any time two rods 

 which unite the ribs and coalesce with one another to form a Sternum, but in some of these Amphibia 

 there originates, in order to supply the place of the Sternum of the higher Vertebrata, a,/ single carti- 

 laginous lamina ; in others, a row of two or three such laminae quite independently of the lateral rays 

 of the vertebral column between the muscles which aid in forming the abdominal walls, and, indeed, 

 close to the middle line of these walls. Now, at first sight, the plastron of the Chelonians appears to be 

 related in its mode of development, on the one hand, in part with the Sternum of the higher 

 Vertebrata, and on the other in part with that of the Batrachia. For, considering it according to the 

 relations which I have observed in various young Tortoises, it originally consists in greatest part, like 

 the Sternum of the higher Vertebrata, of a few cartilaginous rods, divided into lateral halves ; and in 

 a smaller part, as in the case of the Sternum of Toads, of a simple single plate lying in the centre 

 (middle line) of the abdominal wall. These various parts, however, become in time connected with 

 one another, and in many Tortoises, after the bony plates originating in them have much enlarged, 

 they form only a single plate. 



But on closer inspection it appears that the various pieces constituting the Plastron of the 

 Chelonians can neither be regarded as equivalent to the Sternum of the higher Vertebrata, iior to that 

 of the Batrachia. The lateral pieces, or the longitudinal rods, are connected with the ribs immedi- 

 ately, neither in the embryos, nor in the young, nor in the adult; but in many, especially in the young 

 of Sphargis, they are situated at a considerable distance from the ribs, whilst in others the so-called alse 

 with which the pieces most nearly approximate the ribs, are obviously only prolongations sent out 

 towards them. There can be little doubt, therefore, that these parts take their origin independently of 

 and at a distance from the ribs, and thus, in regard to their mode of development, are very different 

 from the Sternum of the higher Vertebrata. Further, according to the observations I have made in 

 Sphargis and Chelonia, two pairs of such longitudinal rods are formed, which for a considerable period 

 (perhaps always in Sphargis) remain far separated from one another ; of these, one is situated in 

 front, the other behind the umbilicus, so that the Plastron of these animals do not develop from 

 before backwards, but from the growing together of parts of which some originally belong to the 

 posterior half of the body. On the other hand, the basis of the Sternum of the higher Vertebrata, 

 especially of Mammals and Birds, consists of only a single pair of longitudinal rods, occupying a 

 position completely in front of the umbilicus, and there is this additional circumstance which consti- 

 tutes a still stronger argument against the view of there being any homology between the rod-shaped 

 bases of the Plastron of the Chelonians and those of the Sternum of the higher Vertebrata, the relation 

 in which the two anterior stand to the " musculi pectorales majores." In Birds, for instance, and the 

 Mammalia, in whom these muscles are formed at about the same time as the two long rods which 

 indicate the commencement of the Sternum ; attached at one of their ends to the external surface of 

 these rods, and thus to some extent, at least, lie beneath them. They are also, at first, like the rods 

 themselves, situated at a considerable distance from one another, and in common with them gradually 

 approximate to one another. 



On the contrary, in the Chelonians these muscles always lie on the upper surfaces of the rods in 



