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