THE INTERLUDE OF CANCER 135
critical
period in a fish or mammal or man is that at which the embryonic organs as a
whole first begin to function. The fish begins to feed itself, digesting the
yolk by intestinal digestion. The mammal or human embryo begins to do the like
(in the absence of food-yolk) by means of the commencing functional activities
of the allantoic placenta. At this epoch in the fish the pancreas gland
manifests its activities by the presence of abundant zymogen granules in the
cytoplasm of its cells. That these result in the secretion of pancreatic
ferments is shown by the digestion of yolk within the gut. Owing to this
digestion, the fish, like the mammal, gets ever bigger and bigger. None of the yolk
enters its stomach, for this has then as little functional activity as the
stomach of a mammal has during foetal life. An internal yolk-sac is formed for
the reception of the yolk from the external one, and the yolk-duct opens into
the duodenum. This fact alone indicates to the embryologist that the pancreas
gland is functioning. In an average marsupial at the critical period this gland
certainly begins its functional activities, for the animal is then born, begins
its mammary nutrition, and digests the milk. If a certain thing happen at the
critical period of a fish, or a marsupial, I know from experience that
something corresponding to it will take place at the like period in a higher
mammal or a man. A fish forms its anus at this period, so does a marsupial,
while in the act of being born, and so does a man, although he does not need it
for some seven months more. As the pancreas gland begins its functions in a
fish or an average marsupial, so it must do in the development of a man.
Otherwise there would be no essential unity in the mode of the development.
Undoubtedly, under the action of the pancreatic ferments, the asexual
structures of a fish development begin