330 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1052 



research on this phase of the problem has been conducted at the De- 

 partment of Embryology of the Carnegie Institution of Washington, 

 D. C, led by the late George Streeter and followed by specialized 

 workers not only at the Institution but throughout leading universi- 

 ties of this country. By careful observation under rigidly controlled 

 techniques in cooperation with gynecologists and the obstetricians, 

 scientists studying the development of the human organism have 

 mapped man's structural development with a surprising degree of 

 accuracy. The advances here in embryology have been tediously slow ; 

 they depend naturally upon securing embryos at an exact known date 

 after fertilization. These developmental changes have been expressed 

 in three-dimensional models, representing young human embryos 

 ranging from a few hours to 21 days after fertilization. 



EXPERIMENTAL EMBRYOLOGY 



Despite the advancement in our knowledge of structural develop- 

 ment, it became apparent during the latter half of the nineteenth 

 century that the descriptive and comparative approach was quite in- 

 adequate for a better understanding of the developmental process. 

 Though descriptive embryology could show in detail how such struc- 

 tures as the brain and the ear develop, it failed to throw any light 

 on the causes that led to their formation and later differentiation. 

 Hence, the causal-analytical approach was applied to embryological 

 development, and experimental embryology came into being. 



Wilhelm Koux (1850-1924), a German anatomist, became the fore- 

 most advocate of this new approach. Hans Spemann (Germany) and 

 Koss G. Harrison (United States of America) soon took over the 

 leadership in experimental embryology. Spemann in 1935 won the 

 Nobel prize for his discovery of the organizer. 



The basic problem of the experimental embryologist is one that con- 

 cerns the concepts of determination. This new generation of biologists 

 continuously seeks the answer to such questions as : Which factors de- 

 termine the origin and further development of a given part of the egg 

 or embryo ? What effect does external environment have upon devel- 

 opment? Actually, in such questions one is asking : Why and how do 

 cells differ in their ultimate fate? The mechanism of organ determin- 

 ation has been carefully analyzed by the experimental embryologist, 

 but it is doubtful if the whole answer will be found here. The classi- 

 cal example is the determination of the lens by the eye vesicle. It was 

 found, principally by the Spemann school, that the optic vesicle, an 

 outgrowth of the brain, comes into contact with the overlying head 

 epidermis and stimulates the latter to form a lens vesicle. When the 

 optic vesicle is removed, no lens is formed. The induction of a lens 



