in the most different creatures. And this leads to the question whether at the beginning of their development all animals are essentially alike, and referable to one common ancestral form. Considering that the "germ" (which at a certain stage appears in the shape of a hollow globe or bag) is the undeveloped animal itself, we are not without reason for assuming that the common fundamental form is that of a simple vesicle, from which every animal is evolved, not only theoretically, but historically.'
This statement is all the more wonderful when we consider that the cells, the all-composing individual units, were not discovered until ten years later.
In 1829 Baer discovered the human egg, and later the chorda dorsalis. In an address delivered in 1834, entitled 'The Most Universal Law of Nature in all Development,' he explained that only from a most superficial point of view can the various species be looked upon as permanent and immutable types;
