FRANCIS O. SCHMITT, Ph.D., D.Sc. 



Professor of Biology, Massachusetts Insti- 

 tute of Technology, Cambridge. 



Contributors 

 RAY L. WATTERSON, Ph.D. 



Associate Professor of Biology, Nortli- 

 western University, Evanston, Illinois. 



ISAAC SCHOUR, D.D.S., Ph.D. 



Professor of Histology, University of Illi- 

 nois, College of Dentistry, Chicago. 



H. B. STEINBACH, Ph.D. 



Professor of Zoology, University of Min- 

 nesota, Minneapolis. 



CURT STERN, Ph.D. 



Professor of Zoology, University of Cal- 

 ifornia, Berkeley. 



VICTOR C. TWITTY, Ph.D. 



Professor of Biology, Stanford University, 

 Stanford, California. 



ALBERT TYLER, Ph.D. 



Professor of Embryology, California In- 

 stitute of Technology, Pasadena. 



PAUL A. WEISS, Ph.D., M.D. (hon.) 



Member, Rockefeller Institute for Med- 

 ical Research, New York, New York; 

 formerly Professor of Zoology, Univer- 

 sity of Chicago. 



BENJAMIN H. WILLIER, Ph.D., Sc.D. 



Henry Walters Professor of Zoology, The 

 Johns Hopkins University, Baltimore, 

 Maryland. 



C. L. YNTEMA, Ph.D. 



Professor of Anatomy, State University of 

 New York Medical Center at Syracuse. 



EDGAR ZWILLING, Ph.D. 



Associate Professor of Animal Genetics, 

 Storrs Agricultural Experiment Station, 

 University of Connecticut, Storrs. 



