GENERAL PHYSICAL CONSTITUTION OF LIVING MATTER 31 



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 hmit 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 (Strongylocenirotus lividus) 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- fig. 6.— after boveri. 

 vide for a definite arrangement or structure of structure of the unfertilized 



° egg of a sea urchin. The 



this material. We shall see later on that this contents of the egg are 



. . 1 . . 1 1 111- divided into three dis- 



structure may be very simple and capable of tinct layers. 

 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, that the network 

 does not occur in living protoplasm, but is caused by the coagulation 

 of the colloids contained in the cells and Hquids of the tissues. It is 

 a fact that the proteins which are dissolved in the Hving body are pre- 

 cipitated by the fixing reagents of the histologists, and when they are 

 precipitated they form net structures which do not exist during life 

 when the proteins are held in solution, f 



* Boveri, Die Polar itat des Seeigeleies. Verhandl. der physik.-med. Gesellsch., Wurzburg, 

 Vol. 34, 1901. t Hsiidy, /our. of Physiology, Vol. 24, p. 158, 1899. 



