THE PROTOPLASMIC SYSTEM 



same way, of course, the growing animal after having been 

 hatched from the egg manufactures its peculiar kind of 

 protoplasm from the food which it takes in. So also yolk 

 is used up in the development of other large eggs, as those of 

 fishes and amphibians; in smaller eggs, however, develop- 

 ment proceeds to the stage of hatching without the utiliza- 

 tion of yolk. For example: in the Nereis egg, yolk is not 

 used during cleavage by the blastomeres; it goes into the 

 four macromeres which form the gut. A sea-urchin egg, in 

 which the yolk has been displaced by centrifugal force, 

 develops perfectly. And yolk-free fragments of marine 

 eggs develop normally as stated above. 



In the egg of Nereis the yolk spheres are polyphasic, i.e., 

 they contain a protein-skein and lipin. If these eggs are 

 placed in hypotonic sea-water, oil in minute drops escapes 

 from the yolk-spheres, leaving a reticular and more watery 

 medium, the whole enclosed by a strong membrane. The 

 yolk spheres now resemble those present normally in the egg 

 of Platy?iereis. On transfer to normal sea-water the oil 

 re-enters the yolk-spheres and their original optical proper- 

 ties are restored.^ The process is thus reversible. An 

 observation like this shows how yolk may vary and that 

 we can not satisfy ourselves with only a rough identification 

 of it as that by staining. Certainly, what thus we 

 identify as yolk is not a pure lipin. 



The Golgi-bodies or Golgi-apparatus, commonly a net of 

 fibres or a cluster of granules of varying form and size, 

 have been described for almost every type of animal and 

 plant cell. Their meaning and function are yet to be 

 made clear. Undoubtedly in some cases what have been 

 described as Golgi-bodies are drops of fat;'^ in other cases 

 even vacuoles have been described as Golgi-apparatus. 



^ Just, 1926. 

 - J'ist, 7927a. 



