468 ANIMAL BIOLOGY 



discrimination of the chief developmental stages, such as cleavage 

 of the egg, germ layer formation, tissue and organ differentiation ; 

 the insistence on the importance of the facts of development for 

 classification; and the discovery of the egg of Mammals. His obser- 

 vations on the origin and development of the germ layers, which 

 afforded the key to many general problems of the origin of the 

 body form, and his emphasis on the resemblance of certain embry- 

 onic stages of higher and lower animals, were made by his suc- 

 cessors, under the influence of the evolution theory, the point of 

 departure for the development of the germ layer theory and the 



RECAPITULATION THEORY. (FigS. 176, 235, 304.) 



From every point of view von Baer created an epoch in embry- 

 ology just when the cell theory began to exert its influence on 

 biological research, and thenceforth it 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 generation 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 embryologists have not been 

 content to employ merely the descriptive method, and the dom- 

 inant 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 environ- 

 mental stimuli, and so on. (Figs. 162, 164, 178.) 



6. Genetics 



The study of inheritance could be little more than a groping 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 between successive 

 generations. Indeed, the present science of genetics has a history 

 largely confined to this century. 



Although clearly intimated by a number of workers, the concep- 

 tion of the continuity of the germ plasm was first forced upon the 

 attention of biologists and given greater precision by Weismann 



