THE BIOLOGY OF THE CELL SURFACE 



If eggs of Arbacia be exposed to hypotonic sea-water dur- 

 ing the time after fertiUzation when the vitelline membrane 

 is being separated and are then returned to normal sea- 

 water, they develop throughout the cleavage-period much 

 as untreated fertilized eggs. A sharp difference is noted, 

 however, in the blastula stage, for then, whereas the normal 

 blastula is made up of cylindrical cells enclosing a cavity, 

 the eggs which have had treatment with hypotonic sea- 

 water are made up of cells approaching the cuboidal form; 

 the cilia of their cells are longer than those on the normal 

 blastula. 



If the eggs of the same species of sea-urchin are treated 

 three minutes after fertilization, that is, two minutes later 

 than those used in the observation described above, part 

 of the egg protrudes beyond the membrane and these eggs 

 develop as twin embryos. In some cases the twins remain 

 joined, in others they become separated. I have found by 

 exposing eggs from the same females at different intervals 

 of fifteen or thirty seconds after fertilization that the most 

 favorable time for the production of these twins comes 

 immediately following the separation of the membrane 

 from the entire egg. Thus, the same treatment if given in 

 a different stage of the egg's development, yields a different 

 result. This statement holds generally for the different 

 periods during the cleavage-process; to treatment with 

 hypotonic sea-water, eggs in different stages of development 

 respond differently as shown by the difference in the types 

 of embryos resulting. 



Now the variety of the results obtained in these observa- 

 tions can be related to ectoplasmic behavior. Beginning 

 with the moment of fertilization the ectoplasm goes through 

 changes in behavior which are easily followed under the 

 microscope. One needs, therefore, only to treat the Qgg 

 during periods when by observation one notes a difference 

 in the quality of ectoplasmic behavior in order to obtain an 

 alteration in the development. 



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