Numerous examples, which might easily be
augmented, of such profound differences, are furnished by Leydig's
'Lehrbuch der Histologie.' In the Crustacea the ovarian eggs actually
sometimes furnish excellent characters for the discrimination of species
of the same genus; thus, for example, in one Porcellana of this country
they are blackish-green, in a second deep blood-red, and in a third dark
yellow; and within the limits of the same order they present
considerable differences in size, which, as Van Beneden and Claus have
already pointed out, stands in intimate connexion with the subsequent
mode of development.
"The organs of the body are formed in the sequence of their organic
importance; the most essential always appear first." This proposition
might be characterised a priori as undemonstrable, since it is
impossible either in general, or for any particular animal, to establish
a sequence of importance amongst equally indispensable parts. Which is
the more important, the lung or the heart--the liver or the kidney?--the
artery or the vein? Instead of giving the preference, with Agassiz, to
the organs of animal life, we might with equal justice give it to those
of vegetative life, as the latter are conceivable without the former,
but not the former without the latter. We might urge that, according to
this proposition, provisional organs as the first produced must exceed
the later-formed permanent organs in importance.
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