THE INTERLUDE OF CANCER 129
the history of which the sexual generation or individual is but an incident.
Another important question to be solved some fifteen years ago was the how and the when of the suppression of the asexual generation. This latter, whether represented by the transient nervous apparatus and other structures of a fish, or by the trophoblast of a mammal, went on flourishing for a certain—not very long—space of time, and then, quite suddenly, all growth was stopped, and its degeneration was initiated. In years long gone by how often have I not watched these asexual structures under the microscope, seen them flourish and blossom, and then—subito, as the Italians say—begin to fade away, as though blighted! The correlation of phenomena is often of the greatest importance to the embryologist in his work; and when this sudden fading away was first established, it was also noted that the commencing formation of the posterior fissure of the spinal cord was a concomitant phenomenon. This led to one of the many little research excursions I have made right up the back-boned series to the mammals, and to the study of human embryos themselves. A whole array of interesting and connected events was soon unearthed, and the putting together of these culminated in the discovery of the critical period—one of the most momentous finds ever made!
“There is a period in the development of every vertebrate embryo, during which, and only then, it resembles the embryo of any other vertebrate in a corresponding phase in certain general features. But while it thus agrees exactly with any other embryo of this period in characters, which are common to all vertebrate animals, it differs from the embryo of any other class in certain special class features, and also from any other embryo