From the Tissue Culture Laboratory of Pathology, Harvard Medical School, and the Pathological Laboratories of the Boston Lying-In Hospital and Free Hospital for Women, Brookline, Mass.

^*Present address: Department of Obstetrics and Gynecology, Tokyo Medical College Hospital, Shinjuku, Tokyo, Japan.

Supported by research Grant C-2451 (Path) from the U. S. Public Health Service.

The authors wish to thank Miss Lydia Brownhill, Mr. John Howard, Miss Harriet McKelvey, and Miss Vanessa Noble for their technical assistance. Mr. Leo Goodman of the Boston City Hospital for his excellent photomicrographs, Miss Marilyn Costello for preparation of the graph in Fig. 1, Dr. Kurt Benirschke and the staff of the Boston Lying-In Hospital for providing trophoblast and technical facilities, and Dr. Roy E. Crowder for his many helpful suggestions.