July, 1891. 
ANIMAL PEDIGREES. 
151 
recapitulatory, for even if small jumps are permissible in 
nature, there is no room for bounds forward of this magnitude. 
Cases of abrupt metamorphosis may always be viewed as due 
to secondary modifications, and rarely, if ever, have any 
significance beyond the particular group of animals concerned. 
For example, a Pluteus larva may be recognised as belonging 
to the group of Ecliinoidea before the adult urchin has com¬ 
menced to be formed within it, and the Lepidopteran cater¬ 
pillar is already an unmistakable insect. Hence, for the ex¬ 
planation of the metamorphoses in these cases it is useless to 
look outside the groups of Ecliinoidea and Insecta respectively. 
Abrupt metamorphosis is always associated with great 
change in external form and appearance, and in mode of life, 
and very usually in mode of nutrition. A gradual transition 
in such cases is inadmissible, because in the intermediate 
stages the animal would be adapted to neither the larval nor 
the adult condition ; a gradual conversion of the biting mouth 
parts of the caterpillar to the sucking proboscis of a moth 
would inevitably lead to starvation. The difficulty is evaded 
by retaining the external form and habits of one particular 
stage for an unduly long period, so that the relations of the 
animal to the surrounding environment remain unchanged, 
while internally preparations for the later stages are in progress. 
Cinderella and the princess are equally possible entities, 
each being well adapted to her environment. The exigencies 
of the situation do not permit, however, of a gradual change 
from one to the other : the transformation, at least as regards 
external appearance, must be abrupt. 
Embryology supplies us with many unsolved problems, 
and it is not to be wondered at that this should be the case. 
Some of these may fairly be spoken of as mere curiosities of 
development, while others are clearly of greater moment. I 
do not propose to catalogue these, but will merely mention 
one which I happen to have recently run my head against, 
and remember vividly. 
The solid condition of the oesophagus in dogfish embryos, 
first noticed by Balfour, is a very curious point. The 
oesophagus has at first a well-developed lumen, like the 
rest of the alimentary canal; but at an early period, stage K 
of Balfour’s nomenclature, the part of the oesophagus over- 
lying the heart, and immediately behind the branchial region, 
becomes solid, and remains solid for a long time, the exact 
date of reappearance of the lumen not being yet ascertained. 
A similar solidification of the oesophagus occurs in tadpoles 
of the common frog. In young free swimming tadpoles the 
oesophagus is perforate, but in tadpoles of about ^ inch 
