260 BULLETIN OF THE 



their shape until the outlines of the mass nearly resemble those of the 

 surface of the tip. This terminal bone is hard externally, and cellular 

 within ; it contains and protects the extremities of the cord and vessels 

 of the column in the positions occupied by them in the embryo before 

 the consolidation was effected. In the different species the number of 

 vertebrae included in this bone varies considerably ; sometimes, also, it 

 is seen to vary in individuals of the same species. Before the appear- 

 ance of the scale-like folds on the tail of the embryo, the skin of the ex- 

 tremity is smooth ; afterward, on some, the tip takes on the semblance 

 of being protected on the sides by scales, the distal portions of which 

 have blended with the cap, while their bases have remained distinct, 

 much as if the conical envelope were still undergoing process of enlarge- 

 ment. In such cases the line of demarcation between the scales and 

 the cap is irregular and indistinct. This condition obtains in species of 

 Tropidonotus, Eutsenia, Nerodia, and allies. The line of separation is 

 very decided and regular in Crotalus. As the tail develops more slowly, 

 the scales do not appear on it until after they are well formed on the 

 body. Up to the time of their formation, the story of the caudal devel- 

 opment of the rattlesnake is the same as that of any other serpent. 

 The general shapes and the numbers of vertebrae differ greatly in the 

 various kinds, but the history is similar in all. With the purpose of in- 

 dicating the manner of growth of the rattle, and, as far as may be, of 

 determining its origin, we shall have to follow it up through different 

 species, a complete series of any one of them not being at hand. 



Sistrurus, Garm. Figs. 1-4. 

 Crotalophorus, Gray, not of Linne. 



S. miliarius, Linn., is the only rattlesnake of which we have a good 

 series of the very early embryos. Some of these, alVeady three inches 

 in length, are not yet furnished with scales on the tail, though tlie 

 entire body is well provided. Outwardly, in these specimens, the tail 

 is short, thick, blunt, slightly compressed, and has no indication of the 

 characteristic feature so prominent after birth. The vertebrae are 

 separate. 



Figures 1 and 2 of the diagrams are drawn from embryos of S. cate- 

 natus, Raf. (Crotabis tergeminus, Say), six and a quarter inches in length. 

 Their only promise of the rattle is to be noted in the shape and size of 

 the cap, or button. Upon the body the scales are perfect ; the button 

 evidently is incomplete, being little more than half of what it ultimately 



