infusorium to man, in one long and continuous series. The erroneous
idea prevailed nearly everywhere that there was one uninterrupted
chain of evolution from the lowest animal to the highest. Cuvier and
Baer proved that this view was false, and that we must distinguish
four totally different types of animals, on the ground of anatomic
structure and embryonic development.

Baer's epoch-making works aroused an extraordinary and widespread
interest in embryological research. Immediately afterwards we find a
great number of observers at work in the newly opened field, enlarging
it in a very short time with great energy by their various discoveries
in detail. Next to Baer's comes the admirable work of Heinrich Rathke,
of Konigsberg (died 1860); he made an extensive study of the
embryology, not only of the invertebrates (crustaceans, insects,
molluscs), but also, and particularly, of the vertebrates (fishes,
tortoises, serpents, crocodiles, etc.). We owe the first comprehensive
studies of mammal embryology to the careful research of Wilhelm
Bischoff, of Munich; his embryology of the rabbit (1840), the dog
(1842), the guinea-pig (1852), and the doe (1854), still form
classical studies. About the same time a great impetus was given to
the embryology of the invertebrates. The way was opened through this
obscure province by the studies of the famous Berlin zoologist,
Johannes Muller, on the echinoderms. He was followed by Albert
Kolliker, of Wurtzburg, writing on the cuttlefish (or the
cephalopods), Siebold and Huxley on worms and zoophytes, Fritz Muller
(Desterro) on the crustacea, Weismann on insects, and so on. The
number of workers in this field has greatly increased of late, and a
quantity of new and astonishing discoveries have been made. One
notices, in several of these recent works on embryology, that their
authors are too little acquainted with comparative anatomy and