I am honored to have been invited to write a foreword for this book, because tumors of the yolk sac have been a preoccupation of mine since the days of my residency, now more than 3 decades ago. At that time, a 3-year-old boy died of a testicular cancer of unknown histo­ genesis. It was bad enough that the child died, but it bothered me even more that medical science did not know the histogenesis of the tumor that destroyed him, and I decided to study testicular cancer. For re­ search training I sought out F. J. Dixon, who had written the Armed Forces Fascicle on testicular tumors. Dr. Dixon and I showed that embryonal carcinoma was a multipoten­ tial malignant stem cell that differentiated into the three embryonic germ layers of murine teratocarcinoma. This led to the idea that the normal counterpart of embryonal carcinoma must also be multi­ potent, and we focused on the preimplantation embryo for the histo­ genesis of the tumor. This idea was strengthened by the discovery that embryonal carcinoma cells made embryoid bodies in the ascites and it was possible to observe the development of these bodies in vitro. These observations led to the idea that embryonal carcinoma was a caricature (gross misrepresentation) of early development, and car­ cinomas in general were a caricature of the process of renewal of their normal counterpart.