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 v. 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 crystallization 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 BischofT, 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. 



II. THE RECAPITULATION 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- 



