REPTILES. 137 



of development) situated immediately beneath the integuments, and most intimately connected with it 

 by one or more layers of a very close connective tissue free from fat, which may be considered as a 

 portion of the subcutaneous connective tissue very different, by reason of its closeness and firmness, 

 from the interstitial cellular tissue, and sharply differentiated from it. On closer investigation it 

 appeared that all the pieces of the Plastron originated in the layer just mentioned, so that these 

 represented their matrix. For, in the embryos of Testudo and Chelonia, and also in the young of 

 Sphargis and Chelonia, they appeared so sunken and enclosed in the layers of the subcutaneous con- 

 nective tissue, that they appeared to be covered even on the superficial surface by a moderately thick plate 

 or layer of this tissue, whilst the clavicles [prce-coracoids] and the muscles which are attached to the bones 

 of the Plastron were only united by means of this strong connective tissue, and were completely separated 

 from the Plastron by it. But the more advanced the stage of development of the young Chelonian, and 

 the greater the thickness of the plates constituting the Plastron, by so much had the superficial or investing 

 layer of connective tissue diminished in thickness, until, at length, it had become quite compressed, and 

 ultimately vanished ; so that at this period each bony plate was ultimately covered on its upper or 

 deep surface by the periosteum, which was again followed by the looser interstitial connective tissue 

 and muscular substance. Moreover, however little developed the Plastron might be, it possessed, in 

 relation to the length of the body, a similar layer, as in the condition of its complete formation, of 

 which the anterior part extended beneath the scaffolding of the Shoulder and its posterior part beneath 

 the Pelvis. But, on the other hand, regarded in relation to the breadth of the body, the ends of its 

 wings stood in the embryo of Chelonia midas (the back of which was far more convex than in the 

 adult) not far from the border of the Carapace, even if it did not absolutely reach it, as in the young 

 and in adult specimens of this species. Hence it may be concluded, that in Chelonia midas the 

 subsequent flattening of the back is essentially occasioned by the circumstance that after the invest- 

 ment of the embryo the ribs gradually extend themselves, and consequently push the margins of the 

 Carapace farther from one another. In three examples of Trionyx the perfectly horizontal alse of the 

 Plastron reached nearly, but not quite, as far outwards as the ribs ; but in the other young Fresh- 

 water Chelonians, in all of which the al<e stand more or less perpendicularly, they reached in 

 Emys Maria, E. europcea, Terrapene tricarinata, and Pentonyx capensis and also in the embryo of 

 Testudo as far as the annular border of the dorsal membrane and the extreme ends of a few ribs, 

 whilst in Platemys the extremities of the anterior alse lay close in front of the thinner halves of the 

 second pair of ribs, and the extremities of the posterior alse close behind the thinner halves of the 

 sixth pair of ribs, and thus on the whole the ends of the alae lay unusually close to the inner part. 



31. As to the significance of the Plastron, and of its several constituent pieces, the majority of 

 anatomists opine that it precisely represents the Sternum of higher animals. Yet Carus and Peters 

 have suggested that it is only equivalent to a portion of the Sternum of higher animals, part of it 

 belonging to the dermal skeleton, and consisting of osseous plates which coalesce with the former. 

 Neither of these views is, however, in my opinion tenable. As regards the latter I have not been 

 able to discover in embryonic or young Chelonians, either here or in the ribs, any special bony plates 

 beneath a somewhat earlier originating part of the Plastron which covered, and had then gradually 

 coalesced with them. The Plastron of Chelonians, the several pieces of which originate near one 

 another upon the same level, would thus, consequently, be admitted to represent, either on one hand 

 the Sternum of other animals, a mere portion of the animal skeleton [endo-skeleton], or, on the other 

 hand, as only a part of the dermal skeleton. My opinion is, for various reasons, upon the whole not 

 favorable to the whole Plastron being equivalent to the Sternum of other Vertebrata ; nor, indeed, to 

 its being a portion of the animal skeleton, but rather that it is a part of the dermal skeleton, npon 

 the following grounds. 



1. According to the researches I have made on the development of the Sternum in Mammals, 

 18 



