M. Rathke on the Development of the Chelonians. 319 



dorsal and the sacral vertebrae, all the rest are developed in the 

 posterior and lateral parts of the circular cutaneous fold (limbus), 

 the anterior portion of which is in great part filled by the ante- 

 rior half of the nuchal disc, which enters progressively into that 

 portion of the circular fold. 



After the flattening of the ventral ribs, there is also, between 

 the teguments and the muscles, in the layer of a thick and solid 

 cellular tissue which joins these different parts, a development of 

 some cartilaginous pieces, of which the plastron is subsequently 

 formed. I have not been able to determine the moment at which 

 their formation commences. The inconsiderable development of 

 the plastron in the oldest embryos, and in the individuals scarcely 

 hatched, leads me to conjecture that it is hardly formed before 

 the middle of the embryonic life, and at all events relatively later 

 than the sternum of birds and mammals. The cartilaginous 

 pieces themselves, appearing as the foundation of the plastron, 

 are at first, for the most part, simple bands, very narrow and 

 thin, forming two pairs, situated one before and the other behind 

 the umbilical aperture. Between these two pairs a very consi- 

 derable space still exists at the period of hatching. Moreover, 

 there is generally formed, or at least in most of the Chelonians 

 (excepting the Sphargis ?), between the anterior extremities of the 

 two even front pieces, a small odd or fifth cartilaginous plate. 

 Subsequently other numerous osseous pieces are developed in these 

 different cartilages, commonly or perhaps always nine in number. 

 Their respective size is very variable, according to the different 

 species of Chelonians ; for, either they grow so much one before 

 another that they meet at their corresponding margins, through- 

 out their length, so as to form a perfectly united plastron, or their 

 growth continues more limited, and then they form a plastron 

 open in the centre, or merely a narrow ring, as is probably the 

 case with the Sphargis. Moreover, the development of the plas- 

 tron differs also in the fact, that its circumference, and especially 

 its length, become relatively greater in some species of Chelo- 

 nians than in others. It then passes beyond the neck and the 

 tail below forming an elongation clothed by the chorion alone, 

 whilst this elongation is wanting in other species. This difference 

 probably depends on the previous existence or not on the ventral 

 side of the body, below and before the anterior feet, and below 

 and behind the posterior feet, of a transversal fold of the chorion, 

 into which the growing plastron might enter. Thus it is pro- 

 bable that the species which exhibit the elongation just described 

 are those in which such a fold already existed. This fact is ren- 

 dered very probable by the examination of the Chelonians of the 

 genus Trionyx, in which this fold is in fact found; but it is 



