EPOCHS IN BIOLOGICAL HISTORY 403 



origin and development of the germ layers, which afforded 

 the key to many general problems of the origin of the body- 

 form (morphogenesis) , and his emphasis on the resemblance of 

 certain embryonic stages of higher animals to the adult stages 

 of lower forms, were crystallized by his successors, under the 

 influence of the evolution theory, as the germ layer theory 

 and the recapitulation theory. 



From every point of view von Baer created an epoch in 

 embryology synchronous with the formulation of the cell 

 theory by Schleiden and Schwann, and it thenceforth became 

 the problem of the embryologist to interpret development in 

 terms of the cell. It is unnecessary to follow historically the 

 establishment of the fact that the egg and the sperm are 

 really single nucleated cells; that fertilization consists in the 

 fusion of egg and sperm and the orderly arrangement of their 

 chief nuclear contents, or chromosomes; that the new genera- 

 tion is the fertilized egg, since every cell of the body as well as 

 every chromosome in every cell is a lineal descendant by 

 division from the zygote, and so from the gametes which 

 united at fertilization to form it. Such, however, are the 

 chief results of cytological study since von Baer. But em- 

 bryologists have not been content to employ merely the de- 

 scriptive method, and the dominant note of the most modern 

 research is physiological the experimental study of the 

 significance of fertilization, the dynamics of cell division, the 

 basis of differentiation, the influence of environmental 

 stimuli, and so on. 



6. Genetics 



The study of inheritance could be little more than a grop- 

 ing in the dark until embryology, under the influence of the 

 cell theory, afforded a body of facts which clearly indicated 

 that typically the fertilized egg is the sole bridge of continuity 



