INTRODUCTION 3 
searches of Haller and his pupil Kuhlemann seemed to establish 
a view which banished all possibility of a rational explanation 
of development, viz., that, in the highest group of animals (the 
mammalia) the embryo arose after fertilization out of formless 
fluids. 
In 1827 vy. Baer discovered the mammalian ovum within the 
Graafian follicle. But no correct interpretation of this discovery 
was possible until the establishment of the cell-theory by Theo- 
dore Schwann in 1839; Schwann concluded as the result of his 
investigations that there was one general principle for the forma- 
tion of all organisms, namely, the formation of cells; that “the 
cause of nutrition and growth resides not in the organism as a 
whole, but in the separate elementary parts, the cells.”’ He 
recognized the ovum as a single cell and the germinal vesicle as 
its nucleus. But on account of his erroneous conception of the 
origin of cells as a kind of erystallization in a primordial sub- 
stance, the cytoblastema, he was unable to form the conception 
of continuity of generations which is an essential part of the 
modern cell-theory. 
Schwann’s theory as regards the ovum was not at once ac- 
cepted. Indeed, for a period of about twenty years some of 
the best investigators, notably Bischoff, opposed the view that 
the ovum is a single cell, and the so-called germinal vesicle its 
nucleus. It was not, indeed, until 1861 that Gegenbaur deci- 
sively demonstrated that the bird’s ovum is a single cell. Even 
after that it was maintained for a long time by His and his fol- 
lowers that all the cells were not derived from the ovum directly, 
but that certain’ tissues, notably the blood and connective tissues, 
were to be traced to maternal leucocytes that had migrated into 
the ovum while it was yet in the follicle. This view was decisively 
disproved in the course of time. 
Il. Tuer RecaAPITuLATION THEORY 
Haeckel’s formula, that the development of the individual 
repeats briefly the evolution of the species, or that ontogeny is 
a brief recapitulation of phylogeny, has been widely accepted by 
embryologists. It is based on a comparison between the embry- 
onic development of the individual and the comparative anatomy 
of the phylum. The embryonic conditions of any set of organs 
of a higher species of a phylum resemble, in many essential par- 
