872 A Sketch of Comparative Embryology. {|December, 
of caterpillars, are only apparent not real exceptions, for in the 
caterpillar the chrysalis is gradually formed, and when perfected 
is merely uncovered by the casting off of the caterpillar skin, 
which masked the changes going on within, and so also the 
opaque crust of the chrysalis conceals the butterfly being formed 
underneath. In some animals, however, the vwészb/e changes 
though still gradual are more rapid at one time than another, as 
when the larval starfish (Brachiolaria) passes in a few hours into 
the adult form. The explanation of the gradualness of develop- 
ment in the Metazoa, is the dependence of the process on altera- 
tions in the single cells, and as these are small and change slowly, 
the whole effect is produced imperceptibly; we notice only that 
the embryo has advanced since we examined it before, we cannot 
see it advancing. 
Now, the construction of an animal out of the cells derived 
from the impregnated egg, depends on two things; frst, the 
arrangement of the cells in relation to each other; second, altera- 
tions in the characters of the cells themselves. We have already 
seen that in the course of segmentation the cells become arranged 
in two layers, the ectoderm and entoderm, both consisting of a 
single stratum of cells, and later there is a set of cells, the eso- 
derm, in between, Fig. 20. Compare also Fig. 15, p. 248. 
Before proceeding further it is desirable 
to say a few words about the middle ger- 
minal layer. Concerning its origin we 
have but little satisfactory information. In 
the lower animals (Radiates) it arises from 
cells which break away from the two 
primitive layers. In the jelly fishes it 
hardly exists as a distinct part, but as the 
20.—Transverse sec- 
tion through the head of em. Brothers Hertwig have shown, is rather an 
ae eee Suid gees incompletely separated portion of the ecto- 
derm. In the Bilateralia, or all animals 
except sponges and radiates, the mesoderm is always present as 
a distinct layer, which is formed after both the ectoderm and 
entoderm. Its exact origin has never been definitely settled, 
although the question has been interminably discussed, especially 
as regards vertebrates. It is, however, known that in some forms 
there are two special cells, one at each side of the primitive mouth 
of the gastrula, distinguished by their large size and containing a 
