The science & philosophy of the organism / by Hans Driesch.

  • Driesch, Hans, 1867-1941.
Date:
1929
Catalogue details

Licence: In copyright

Credit: The science & philosophy of the organism / by Hans Driesch. Source: Wellcome Collection.

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    its head goes in. The moment that the head has penetrated the protoplasm of the egg a thin membrane is formed by the latter. This membrane is very soft at first, becoming much stronger later on; it is very important, for all experimental work, that by shaking the egg in the first minutes of its existence the membrane can easily be destroyed without any damage to the egg itself. And now occurs the chief phenomenon of fertilisation : the nucleus of the spermatozoon unites with the nucleus of the egg. When speaking of maturation, we mentioned that half of the chromatin was thrown out of the egg by that process : now this half is brought in again, but comes from another individual. It is from this phenomenon of nuclear union as the main character of fertilisation that almost all theories of heredity assume their right to regard the nuclei of the sexual cells as the true “ seat ” of inheritance. Later on, we shall have occasion to discuss this hypothesis from the point of view of logic and fact. After the complete union of what are called the male and the female “ pronuclei ”, the egg begins its development; and this development, in its first steps, is simply pure cell- division. We know already the chief points of this process, and need only add to what has been described that in the whole first series of the cell-divisions of the egg, or, to use the technical term, in the whole process of the “ cleavage ” or “ segmentation ”, there is never any growth of the daughter- elements after each division, such as we know to occur after all cell-divisions of later embryological stages. So it happens, that during cleavage the embryonic cells become smaller and smaller, until a certain limit is reached ; the sum of the volumes of all the cleavage cells together is equal to the volume of the egg. But our future studies will require a more thorough knowledge of the cleavage of our Echinus. The first division plane, or, as we shall say, the first cleavage plane, divides the eggs into equal parts ; the second lies at right angles to the first and again divides equally : we now have a ring of four