"
Then, after Johannes Muller has combated the idea of a graduated scale
of animals, and of the passage through several animal grades during
development, he continues:--"What is true in this idea is, that every
embryo at first bears only the type of its section, from which the type
of the Class, Order, etc., is only afterwards developed."
In 1856, in an elementary work,* (* 'Principles of Zoology' Part 1
Comparative Physiology. By Louis Agassiz and A.A. Gould Revised Edition
Boston 1856.) in which it is usual to admit only what are regarded as
the assured acquisitions of science, Agassiz expresses himself as
follows:--
"The ovarian eggs of all animals are perfectly identical, small cells
with a vitellus, germinal vesicle and germinal spot" (paragraph 278).
"The organs of the body are formed in the sequence of their organic
importance; the most essential always appear first. Thus the organs of
vegetative life, the intestine, etc., appear later than those of animal
life, the nervous system, skeleton, etc., and these in turn are preceded
by the more general phenomena belonging to the animal as such"
(paragraph 318). "Thus, in Fishes, the first changes consist in the
segmentation of the vitellus and the formation of a germ, processes
which are common to all classes of animals. Then the dorsal furrow,
characteristic of the Vertebrate, appears--the brain, the organs of the
senses; at a later period are formed the intestine, the limbs, and the
permanent form of the respiratory organs, from which the class is
recognised with certainty.
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