M. Rathke on the Development of the Chelonians. 321 



an anatomical point of view, it has nothing in common with the 

 sternum of other animals. This supposition once admitted, we 

 may refer the situation of the bones of the shoulder and the basin 

 of adult Chelonians to the relations existing in other animals. 

 There is then no longer anything extraordinary in the arrange- 

 ment of these parts, but only something specific produced by the 

 curious development of the dorsal parts of the body. With re- 

 spect to the position of the scapulae, they are situated before 

 the ribs in older embryos and in young Chelonians, and it is 

 more than probable that they occupied this position even before 

 the development of the ribs had made any sensible progress, and 

 that they were not merely protruded by the ribs in consequence 

 of the rapid growth of the body in width. In fact, the first pair 

 of ribs, near and a little in advance of which they are situated in 

 older embryos and young individuals, is scarcely remarkable 

 either for its very great length or its width ; it is on the contrary 

 extremely short and thin, so that a displacement of the scapulae 

 becomes impossible. Moreover, we see in some fishes, some 

 Saurians (Titigna sincoides), and even in a mammal (Ornitho- 

 rhynchus), the scapulae occupying a similar position in front of 

 the ribs. In the Didelphis virginiana, the whole scapula, or at 

 least the lower part with the scapular articulation, is situated an- 

 terior to the ribs, and it thus becomes probable that in these ani- 

 mals also, at least in a period previous to their development, the 

 entire scapula, before it acquires its oblique position and its 

 considerable width, is situated in front of the ribs. In other 

 mammals the scapulae (although they are never so protruded 

 as in the Chelonians and the Ornithorhynchi) meet in the first 

 period of their development much further in advance than in the 

 subsequent periods. In the hog, for instance, the scapula, a 

 little after the formation of the anterior foot, covers the two an- 

 terior ribs of the corresponding side. When it is not perceptible 

 as a separate part, it does not at the commencement go beyond 

 the first rib, whilst it extends from the first up to the seventh in 

 adult hogs. 



Lastly, the direction of the scapulae in the Chelonians does 

 not differ much from that which is observed in the Ornithorhynchi 

 and several Saurians, in which they also occur perpendicular. 

 Their situation below the osseous parts of the back in the adult 

 Chelonians is produced subsequently by the successive develop- 

 ment, for even in the oldest embryos they are in immediate con- 

 tact, by their upper extremities, with the layer of the subcuta- 

 neous cellular tissue. 



The metamorphosis which I have just described results from 

 the considerable expansion of the second pair of ribs, extending 



Ann. jf Mag. N. Hist. Vol. xviii. 2 A 



