318 M. Rathke on the Development of the Chelonians. 



with the neighbouring ribs differ much from those of the inter- 

 mediate ribs ; for, as the latter increase greatly in width, the se- 

 cond exceeds the first, and the last but one surpasses the last so 

 much that it covers it more or less entirely. Soon after the eight 

 pairs of intermediate ribs have begun to widen, a branch protrudes 

 superiorly from each rib, near the vertebral column. This branch 

 continually increasing passes beyond the rare and thin dorsal 

 muscles ; the two sacro-spinal muscles (situated on the summit of 

 the ribs, throughout the length of the body) unite with the spinal 

 apophysis of the vertebrae of the same rib, and become quite as 

 wide as the body itself of the rib. The spinal apophyses make their 

 appearance, even before the hatching upon the second vertebra 

 up to the eighth. They remain very short ; but, contrary to the 

 general laws of development of vertebrated animals, they increase 

 so much in width, after their ossification, that they form at last 

 a series of horizontal plates of the average size. 



I cannot adopt the opinion which supposes these plates to be 

 formed in the subcutaneous cellular tissue, independently of the 

 vertebral column, in the chorion itself or below it; that they 

 unite afterwards with the vertebras, and that the remarkable 

 width of the eight pairs of intermediate ribs is also the result of 

 contact, and subsequently of an adhesion with the osseous plates 

 formed above these ribs. On the contrary, these assumptions 

 are refuted by my observations. 



After the successive expansion of the bodies of the eight pairs 

 of intermediate ribs, of their superior branch, exclusively peculiar 

 to the Chelonians and of the spiny apophyses of the same ver- 

 tebras, an osseous plate is finally formed by the contact and ad- 

 hesion of the corresponding margins of all these parts, composed 

 of numerous pieces, which extends to form the carapace above 

 and covers the viscera. To increase and complete this shell, 

 already very considerable, we observe other osseous plates unite 

 with it. They are formed on the back, wholly independent of 

 the vertebral column and of the ribs, in a very thick and solid 

 layer of the subcutaneous cellular tissue, and must be considered 

 as the external skeleton (cutaneous skeleton) of the animals*. 

 Their number varies in the different species of Chelonians. In 

 the genus Trionyx only one disc is found ; it is situated on the 

 neck immediately in front of the dorsal vertebras. There are also 

 some discs in the posterior margin of the carapace in some spe- 

 cies of Trionyx ; but they remain cartilaginous. Besides this 

 nuchal plate, which is always very large, several small subcuta- 

 neous plates are also developed in most of the Chelonians. 

 Among these, a small number only originate above the last 



* These terms are borrowed from the nomenclature of M. Cams. 



