organs which accomplish the phenomena of animal life--the functions of
sensation and motion, and the covering of the body. From the lower or
vegetative layer come the organs which effect the vegetative life of
the organism--nutrition, digestion, blood-formation, respiration,
secretion, reproduction, etc.

Each of these original layers divides, according to Baer, into two
thinner and superimposed layers or plates. He calls the two plates of
the animal layer, the skin-stratum and muscle-stratum. From the upper
of these plates, the skin-stratum, the external skin, or outer
covering of the body, the central nervous system, and the
sense-organs, are formed. From the lower, or muscle-stratum, the
muscles, or fleshy parts and the bony skeleton--in a word, the motor
organs--are evolved. In the same way, Baer said, the lower or
vegetative layer splits into two plates, which he calls the
vascular-stratum and the mucous-stratum. From the outer of the two
(the vascular) the heart, blood-vessels, spleen, and the other
vascular glands, the kidneys, and sexual glands, are formed. From the
fourth or mucous layer, in fine, we get the internal and digestive
lining of the alimentary canal and all its dependencies, the liver,
lungs, salivary glands, etc. Baer had, in the main, correctly judged
the significance of these four secondary embryonic layers, and he
followed the conversion of them into the tube-shaped primitive organs
with great perspicacity. He first solved the difficult problem of the
transformation of this four-fold, flat, leaf-shaped, embryonic disk
into the complete vertebrate body, through the conversion of the
layers or plates into tubes. The flat leaves bend themselves in
obedience to certain laws of growth; the borders of the curling plates
approach nearer and nearer; until at last they come into actual
contact. Thus out of the flat gut-plate is formed a hollow gut-tube,