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Show 1894.] VERTEBRAL COLUMN OF THE BULL-FROG. 479 The next two pieces (C, D) appear to be entirely normal vertebrae, and separated from the rest would undoubtedly be regarded as such. The third compound piece (E) (see figs. 12-15) consists of the normal eighth vertebra, with an extra half neural arch and transverse process on the right side (VII.'). Ventrally, the centrum is only slightly unsymmetrical, being a little longer on the right than on the left side. There is no sign or mark of an extra centrum. But dorsally there is evidence, in the existence of two neural spines (d, e), together with a deep groove halfway along the right arch, as well as in the presence of two transverse processes, of fusion of an extra half vertebra (VII.'). There is an intervertebral foramen between the two transverse processes of the right side : of the latter the anterior (VII.') is directed straight out; the posterior (VIII.') is flattened, and its distal extremity curves upwards, in a manner only slightly different from the normal. I have used the word " fusion " of vertebrae ; but in the case of the second or fifth vertebral pieces I do not feel at all sure that such is the proper term to employ. Bateson ('Materials for the Study of Variation'), in describing a somewhat analogous case of an extra half vertebra in a Python, remarks (p. 104) that the bone in question (which is closely like the 5th piece (fig. 12) of Rana mugiens) " is not two vertebrae simply joined together, as bones may be after inflammation or the like, but it is two vertebrae whose adjacent parts are not formed, and between which the process of division has been imperfect: with more reason it may be spoken of as one vertebra partly divided into two, but this description also scarcely recognizes the real nature of the phenomenon." Bateson refers to one or two other similar cases in Eeptiles. But in the specimen of Rana mugiens we have this point of difference, that the normal number of vertebrae and transverse processes is retained on each side-there being, however, 9 neural spines to the first eight vertebrae. There is evidently no "intercalation" of half a vertebra; but it seems to me that during development the lines separating the mesoblastic somites were oblique instead of at right angles to the axis of the body, and that the wrong halves met across the middle line, as we find in cases of abnormal segmentation of the body in Chaetopods (see Cori, Buchanan, &c), giving rise to " spiral segments." Thus the sclerotome destined to give rise to the right half of the fourth vertebra has united, not with its corresponding left sclerotome, but with the left sclerotome of the fifth vertebral segment. If this be the case, then the apparently symmetrical vertebral pieces C and D are not really so, but, as is indicated on the drawings, the third piece consists of \ V. and | VI., and the fourth piece of | VI. and | VII.-resulting in two entire, and seemingly normal, vertebrae. But what is as curious as anything is the rectification which occurs in the fifth vertebral piece (E). |
