dants, because each organ possesses its
own specific rate of development. In this way it comes about naturally
that organs which become differentiated rapidly, as, for example, the
medullary tube, as a rule dominate earlier periods of ontogeny than do
the organs of locomotion. For the same reason the cerebral hemispheres
of man are almost as large in youth as in maturity. The picture which an
embryo gives is not a repetition in detail of one and the same
phylogenetic stage; it consists rather of an assemblage of organs, some
of which are at a phyletically early stage of development, while others
are at a phyletically older stage."[536]

A different line of attack was that adopted by O. Hertwig in a series of
papers, which contain also what is perhaps the best critical estimate of
the present position and value of descriptive morphology.[537]

It had not escaped the notice of many previous observers that quite
early embryos not infrequently show specific characters even before the
characters proper to their class, order and genus are developed--in
direct contradiction of the law of von Baer. Thus L. Agassiz[538] had
remarked in 1859 that specific characteristics were often developed
precociously. "The Snapping Turtle, for instance, exhibits its small
crosslike sternum, its long tail, its ferocious habits, even before it
leaves the egg, before it breathes through lungs, before its derm is
ossified to form a bony shield, etc.; nay, it snaps with its gaping jaws
at anything brought near, when it is still surrounded by its amnion and
allantois, and its yolk still exceeds in bulk its whole body" (p. 269).

Wilhelm His,[539] in the course of an acute and damaging criticism of the
biogenetic law as enunciated by Haeckel, showed clearly that by careful
examination the very earliest embryos of a whole series of Vertebrates
could be distinguished with certainty from one another. "An identity in
external form of different animal embryos, despite the common
affirmation to the contrary, does not exist. Even at early stages in
their development embryos possess the characters of their class and
order, nay, we can hardly doubt, of their species and sex, and even
their individual characteristics" (201).

This specificity of embryos was affirmed with even greater confidence by
Sedgwick in a paper critical of von Baer's law.[540] He wrote:--"If v.
Baer's law has any meaning at all, surely it must imply that animals so
closely allied as