462 THEOPHILUS S. PAINTER 



In the living egg, the similarity between normal and monaster 

 eggs may be considered under three heads: the aster, the ecto- 

 plasmic or hyaline layer, and the granular cytoplasm. 



The appearance of the asters, their growth, their changes in 

 form, and finally their retreat towards the surface of the egg 

 are phenomena so well known for the sea urchin egg that they 

 call for no description here. The single aster passes through 

 the same stages, as my description and figures will show. 



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The correspondence in the behavior of the hyaline or ecto- 

 plasmic layer calls for a word by way of explanation first, as 

 this part of the egg does not seem to have attracted the atten- 

 tion which it merits. It has been known for a long time (Sel- 

 enka, '83 was the first to describe it) that there is a thin layer 

 of ectoplasm surrounding the eggs of echinoderms, some mol- 

 luscs, and ctenophores. This layer lies just beneath the fertili- 

 zation membrane and, in the sea urchin at least, does not become 

 prominent until the first division cycle is well advanced. It 

 reaches its maximum development over the whole surface of 

 the egg a few minutes before cleavage sets in. Now if one 

 examines eggs with a dim light just as cleavage is about to begin, 

 one will see that part of this layer has become swollen or blis- 

 tered. This swollen area runs as a narrow girdle around the 



