No. 409.] THE SCULPTURED TORTOISE. 23 
their bodies the germ layers undergo a curious change in posi- 
tion. The tail of a developing tadpole is composed of an outer 
covering of ectoderm — which ultimately gives rise to the outer 
layers of the skin—and of a core of mesoderm. These two 
masses of tissue grow in very different ways, so that as the 
tail lengthens the ectodermic covering, which is most actively 
produced anteriorly, slips posteriorly over the underlying meso- 
derm, whose region of growth is chiefly at the posterior end. 
Although this posterior migration of the ectoderm has been 
actually demonstrated only in the tadpole, there is reason to 
believe that it occurs in other vertebrates. Admitting its 
existence in the turtle, it affords an easy means of explaining 
the conditions described. The scutes of turtles are derived 
from the ectoderm, the bony plates from the mesoderm. In 
the anterior part of the carapace these ectodermic and meso- 
dermic derivatives, according to the peculiarity of growth just 
explained, would not undergo any separation but would retain 
their embryonic positions. Hence, if the material from which 
both scutes and plates arise were modified by any local influence, 
the resultant scutes and plates would be found together, as in 
the first abnormal specimen described. In the posterior part 
of the carapace, on the other hand, the ectodermic migration 
would be excessive and any early local disturbing influence 
that affected both scute and plate-producing tissue would leave 
its trace in the adult in the form of a region of modified scutes 
posterior to a region of modified bony plates, — a condition 
realized in the second abnormal specimen. Thus, from what is 
known of the methods of growth of the integument and subjacent 
parts in vertebrates, it is fair to assume that the abnormalities 
of scutes and bony plates in the second specimen, though 
separated in the adult, may be as truly correlated as those of 
the first specimen, in which the modified areas still remain 
superimposed. 
The older anatomists have very generally pointed out the 
superficial resemblances between the scutes and the bony 
plates of the chelonian carapace, but they have as a rule 
denied any close relation between these two sets of structures. 
Gegenbaur ('98, pp. 132, 174), in his recently published volume 
