THE BIOLOGY OF THE CELL SURFACE 



plasm project.^ By anastomosis of their free ends a very 

 fine covering membrane forms. ^ Thus the hyaline plasma- 

 layer is clear granule-free cytoplasm in the form of finely 

 spun threads covered by a thin membrane. Other eggs, as 

 shown in the chapter, The Ectoplasm, possess these ecto- 

 plasmic filaments. They are present on every marine egg 

 that I know. Unfertilized sea-urchins' eggs treated with 

 strong hypertonic sea-water also show that the surface- 

 cytoplasm is made up of radial strands. Eggs of Nereis 

 and of Platynereis show especially well after fertilization 

 immediately below the vitelline membrane a delicate 

 plasma-membrane, which is the covering film of cytoplas- 

 mic prolongations. It is these prolongations, of greater 

 length and diameter than those of sea-urchins' eggs, which 

 give the surface of the eggs of these worms their striated 

 appearance. These prolongations constitute the outer 

 region of the ectoplasm. They and their covering film 

 compose the hyaline plasma-layer of the egg. 



Others before me have described on the fertilized egg 

 In later stages this layer as made up of filaments. Meves,^ 

 for example, has pointed out that some time after fertiliza- 

 tion the layer is composed of threads and a thin covering 

 membrane. This structure of the hyaline plasma-layer 

 can be demonstrated by placing fertilized eggs in calcium- 

 free sea-water; the covering membrane becomes destroyed 

 leaving the filaments free. 



During the whole cleavage-period, sea-urchins' eggs 

 show on the surface of each blastomere these fine cyto- 

 plasmic projections covered by a delicate membrane which 

 together make a continuous film enclosing the blastomeres. 

 The filaments are most easily seen between adjacent blasto- 

 meres as they are separating at cleavage. Since there is no 



1 Cf. also Berthold, i8S6, and Thfcl, iSg2. 



2 See also Mrs. Andrezvs. 



3 Meves, igi2. 



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