. The dynamics of living matter . FIG. 6. — AFTER BOVERI. Structure of the unfertilized egg of a sea urchin. The contents of the egg are divided into three dis- tinct layers. cannot be used as an unequivocal answer to our question, inasmuch as the possibility exists that in later stages of segmentation the different cells undergo different chemical changes, whereby they no longer remain equal in quality. If we raise the question why such a limit exists in regard to the divisibility of living matter, the answer is possibly given by Boveri's observation that the unsegmented egg of the sea urchin
. The dynamics of living matter . FIG. 6. — AFTER BOVERI. Structure of the unfertilized egg of a sea urchin. The contents of the egg are divided into three dis- tinct layers. cannot be used as an unequivocal answer to our question, inasmuch as the possibility exists that in later stages of segmentation the different cells undergo different chemical changes, whereby they no longer remain equal in quality. If we raise the question why such a limit exists in regard to the divisibility of living matter, the answer is possibly given by Boveri's observation that the unsegmented egg of the sea urchin (Strongylocentrotus limdus] possesses three different layers.* It is possible that these three layers contain chemically different material, and that only those fragments of an egg are capable of development which contain material of each of the three layers. If this be correct, it will certainly not suffice to mix the chemical con- stituents of the egg in order to produce the phenomena of development; but we must pro- vide for a definite arrangement or structure of this material. We shall see later on that this structure may be very simple and capable of a physicochemical definition. The limits of divisibility seem therefore to depend upon the physical structure of the cells or organs. These limits vary for different organisms and cells. The smallest piece of a sea-urchin egg that can reach the plu- teus stage is still visible with the naked eye, and is therefore consider- ably larger than bacteria or many algae, which also may be capable of division. 2. FOAM STRUCTURES AND EMULSIONS Living matter seen through the microscope invariably offers the same characteristic appearance which has caused biologists to desig- nate it with one general term; namely, protoplasm. Yet the common physical features of living "protoplasm" are still a matter of contro- versy. Some authors maintain that the protoplasm is a network of fine fibers, while others say, and apparently justly
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