io THE CELL [CH. 



derm egg as affording an instance in which clearly defined 

 structure could be studied, but E. B. WILSON (1900, p. 25) 

 has shown conclusively that these eggs consist of an emul- 

 sion of larger and smaller fluid droplets in an apparently 

 homogeneous ground-substance. It is impossible to draw 

 any real distinction between the larger of these droplets and 

 the yolk granules of other animals, and although it is certain 

 that these eggs have an alveolar structure (the droplets 

 forming the alveoli) it is hardly possible to regard this 

 structure as representing that of protoplasm, for the proto- 

 plasm, properly speaking, is confined to the apparently 

 homogeneous substance in which the droplets are embedded. 

 CON KLIN finds that in the egg of Crepidula this protoplasmic 

 ground-substance is by no means fluid, but is elastic and 

 contractile, so that when the egg is distorted by being 

 centrifuged it "tends to come back to its normal form and 

 to bring back to their normal positions displaced constitu- 

 ents of the cell 1 ." 



The statement that protoplasm is structureless does not 

 imply that it is a homogeneous chemical compound, for it 

 is undoubtedly a highly complex and unstable mixture; it 

 means that it consists of a number of colloidal substances 

 in solution, and so intimately mixed that no mechanical 

 structure exists which can be detected by the microscope. 

 Moreover, these observations apply only to unmodified 

 protoplasm as seen in an undifferentiated embryonic cell, 

 and even in such a cell there are practically always inclu- 

 sions of various sorts consisting partly of substances pro- 

 duced or secreted by the cell and used by it in its further 

 development ("deutoplasm"), and partly of substances 

 which seem to have a more vital connection with the cell's 

 activity, and which may almost be regarded as cell-organs 

 1 E. G. CONKLIN (1917). 



