THE PERIOD OF EXPERIMENTAL EMBRYOLOGY 7 



2. "After the more general characteristics those that are less general arise 

 and so on until the most special characteristics appear." 



3. "The embryo of any particular kind of animal grows more unlike the 

 forms of other species instead of passing through them." 



4. "The embryo of a higher species may resemble the embryo of a lower 

 species but never the adult form of that species." (This latter state- 

 ment is the basis of the Biogenetic Law when it is properly inter- 

 preted.) 



Following von Baer, Kowalevsky (1866) stated that all animals 

 pass through a gastrula stage. Haeckel (1874) proposed a Gastrea 

 Theory which suggested that the permanent gastrula, the adult 

 Coelenterata, might be the form from which all higher diploblastic 

 (gastrula) stages in their life history were derived. 



The Period of Cellular Embryology 



As the microscope was still further refined, and embryos could be 

 studied in greater detail, it was natural that a further subdivision of 

 the field of embryology would occur. In 1831 Robert Brown dis- 

 covered the nucleus; in 1838 Schleiden and Schwann founded the cell 

 theory; in 1841 Remak and Kolliker described cell division; in 1851 

 Newport observed the entrance of the spermatozoon into the frog's 

 egg; and in 1858 Virchow published his "Cellular Pathology." Later, 

 in 1878, Whitman and Mark initiated the study of cell lineage 

 ("Maturation, Fecundation, and Segmentation of Limax") by which 

 the fate of certain early blastomeres of the embryo was traced from 

 their beginning. In 1882 Flemming discovered the longitudinal 

 splitting of chromosomes; and Sutton in 1901 gave us the basis for 

 the modern chromosomal theory of inheritance. The study of the 

 embryo was thus broken down into a study of its constituent cells; 

 then to the nucleus; and finally to the gene. From the morphological, 

 physiological, or genetic aspects, this field of cellular embryology is 

 still very active. 



The Period of Experimental Embryology 



During the latter part of the nineteenth century, investigators began 

 to alter the environment of embryos, and surgically and mechanically 

 to interfere with blastomeres and other parts of the developing em- 

 bryo. This new "experimental" approach required a prior knowledge 

 of morphological and comparative embryology. It was hoped that, by 



