THE RISE OF EMBRYOLOGY 219 
except the very lowest there arise in the course of devel- 
opment leaf-like layers, which become converted into the 
“fundamental organs” of the body. 
Now, these elementary layers are not definitive tissues of 
the body, but are embryonic, and therefore may appropriately 
be designated ‘‘germ-layers.” The conception that these 
germ-layers are essentially similar in origin and fate in all 
animals was a fuller and later development of the germ-layer 
theory, a conception which dominated embryological study 
until a recent date. 
Von Baer recognized four such layers; the outer and inner 
ones being formed first, and subsequently budding off a 
middle layer composed of two sheets. A little later (1845) 
Remak recognized the double middle layer of Von Baer as a 
unit, and thus arrived at the fundamental conception of three 
layers—the ecto-, endo-, and mesoderm—which has so long 
held sway. For a long time after Von Baer the aim of em- 
bryologists was to trace the history of these germ-layers, and 
so in a wider and much qualified sense it is to-day. 
It will ever stand to his credit, as a great achievement, 
that Von Baer was able to make a very complicated feature 
of development clear and relatively simple. Given a leaf-like 
rudiment, with the layers held out by the yolk, as is the case 
in the hen’s egg, it was no easy matter to conceive how 
they are transformed into the nervous system, the body-wall, 
the alimentary canal, and other parts, but Von Baer saw 
deeply and clearly that the fundamental anatomical features 
of the body are assumed by the leaf-like rudiments being 
rolled into tubes. 
Fig. 67 shows four sketches taken from the plates illus- 
trating von Baer’s work. At A is shown a stage in the forma- 
tion of the embryonic envelope, or amnion, which surrounds 
the embryos of all animals above the class of amphibia. 8B, 
another figure of an ideal section, shows that, long before the 
