320 M. Rathke on the Development of the Chelonians, 



scarcely filled by the parts of the plastron, little developed in 

 this genus. 



A specific and very remarkable feature in the Chelonians is 

 subsequently the relation of their bones of the trunk with the 

 very solid subcutaneous cellular tissue, forming a layer of little 

 thickness and commonly considered cartilaginous. All the os- 

 seous pieces contiguous to this layer, namely, the spiny apophyses 

 of the vertebrae from the second up to the eighth, the eight pairs 

 of intermediate ribs, the supplementary plates of the shell, and 

 often also all the pieces of the plastron, after having lost by re- 

 absorption on their external surface the periosteum, come in con- 

 tact with the subcutaneous cellular tissue. This happens after 

 the hatching of the embryo and principally on the ribs, so that 

 the periosteum disappears gradually, from the upper extremity 

 (nearer to the vertebrae) toward the lower extremity. In the 

 marine Chelonians it does not disappear wholly on the lower 

 extremity, but only as far as the part of the ribs which never 

 expand to any extent. As soon as the osseous substance of this 

 part comes in immediate contact with the subcutaneous cellular 

 tissue, numerous medullary cellules appear in the direction of 

 this tissue, which, at least in the commencement, are externally 

 open. By degrees their number increases considerably, and the 

 bones which I have just named become at the same time stronger 

 and very porous, although there may be, according to the spe- 

 cies, a marked difference in their porosity. The cellules are not 

 principally filled with fat, as is the case in more perfect verte- 

 brated animals, and even in the Chelonians, in the bones further 

 removed from the chorion ; they are filled by the subcutaneous 

 cellular tissue. This tissue enters gradually by the apertures of 

 the cellules as by a radiation of numerous small roots, and col- 

 lects there always in proportion to their growth. Nevertheless 

 the layer of this tissue situated between the bones and the chorion 

 constantly diminishes, not only relatively, but also in part abso- 

 lutely, so that it seems wholly to be wanting on the carapace and 

 the plastron in some Chelonians, especially in the Emys europaa. 



If we consider, as usual, the plastron of the Chelonians as a 

 portion of the nervous skeleton and as the homologue of the ster- 

 num of the other vertebrated animals, we must also admit that the 

 bones composing the scapular and the pelvic arches are situated 

 in a wholly contrary manner to the general disposition of these 

 parts (when they exist) in the other vertebrated animals. They 

 would be situated so as to remain wholly inexplicable, according 

 to our present knowledge of the development of the animals. But, 

 from some facts, I believe I can prove evidently that the plastron 

 is nothing else than a part of the cutaneous skeleton, and that, in 



