1894.] VERTEBRAL COLUMN OE THE BULL-FROG. 479 



The next two pieces (C, D) appear to be entirely normal 

 vertebra?, 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 trans- 

 verse 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 (cl, 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 antei'ior (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 Sana muc/iens) " is not two vertebrae 

 simply joined together, as bones may be after inflammation or the 

 like, but it is two vertebra? 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 Reptiles. 



But in the specimen of Sana mugiens we have this point of 

 difference, that the normal number of vertebra? and transverse 

 processes is retained on each side — there being, however, 9 

 neural spines to the first eight vertebra?. 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 tind in 

 cases of abnormal segmentation of the body in Cha?topods (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 h VI., and the 

 fourth piece of k VI. and | VII. — resulting in two entire, and 

 seemingly normal, vertebra?. But what is as curious as anything 

 is the rectification which occurs in the fifth vertebral piece (E). 



