October 25, 1912] 



SCIENCE 



563 



the nature of the fertilization membrane of 

 the sea urchin egg. The methods which have 

 been used heretofore in the study of this so- 

 •called membrane have been inadequate to 

 determine its exact nature. The writer has 

 made some observations, by the use of meth- 

 ods which are apparently new to this field, 

 •which seem to throw a new light on the 

 structure of the various coatings on this egg. 

 As is well known, there has been no agreement 

 among cytologists concerning the number and 

 nature of the coverings of this egg. A few 

 investigators have recognized the presence of 

 a thick jelly surrounding the egg, but have 

 given us no methods of demonstrating its 

 true extent or nature. Further, no one has 

 used methods by which the physical character- 

 istics of the vitelline membrane could be de- 

 termined in the living egg, and the relation of 

 this membrane to the cytoplasm on the one 

 hand and to the egg-jelly on the other. 



The normal unfertilized egg is covered by a 

 soft invisible jelly about 23 microns in thick- 

 ness. Beneath this and closely stuck to the 

 surface of the cytoplasm is a tough somewhat 

 ■elastic vitelline membrane. Morphologically, 

 this latter is the only membrane on the egg 

 ■of Arbacia. It is literally glued to the outer 

 surface of the cytoplasm. The average of a 

 number of measurements shows that the vitel- 

 line membrane is about 1.9 microns in thick- 

 ness. In the living egg the inner part of this 

 structure is seen as a light line on the outer 

 surface of the cytoplasm when the light is 

 sufficiently stopped down. The refractive in- 

 dex of the outer part is so nearly that of sea 

 water that this portion is invisible. 



In the reaction of the egg to the spermato- 

 zoon, striking morphological changes occur in 

 the vitelline membrane and the surface of the 

 cytoplasm. The change in form of the egg- 

 jelly is slight. About one and one half min- 

 utes after active spermatozoa are mixed with 

 the eggs, a definite swelling of the vitelline 

 membrane occurs. The extent and location 

 of the swelling varies greatly in diiierent 

 eggs. In some eggs the swelling is complete 

 in one minute, but three to five minutes are 

 usually required for the maximum swelling of 



this structure. During the swelling of the 

 vitelline membrane its refractive index is so 

 changed that it usually becomes distinctly 

 visible by the usual microscopical examina- 

 tion. When swelling is complete the thickest 

 portion of this structure frequently measures 

 as much as nine microns. About six minutes 

 after insemination of the eggs the surface of 

 the cytoplasm swells and changes its refrac- 

 tive index. In one minute the swelling is 

 complete and measures about one micron in 

 thickness. This is the well-known hyaline 

 plasma-layer. By the time the swelling of 

 the vitelline membrane has become well ad- 

 vanced, a change occurs in the refractive in- 

 dex of the inner part of the egg-jelly and this 

 becomes visible. As seen by the usual micro- 

 scopical examination the so-called fertilization 

 membrane of the egg of Arbacia consists of 

 three parts, viz., the inner part of the egg- jelly 

 which has undergone a change in its refrac- 

 tive index, the swollen vitelline membrane 

 and the thin highly refractive surface layer 

 of the cytoplasm. This hyaline layer is stiH 

 very adherent to the vitelline membrane. The 

 edematous vitelline membrane is softer and 

 more elastic than it is in the unfertilized egg. 

 If this structure is partially dissected from 

 the fertilized egg it frequently contracts to a 

 glutinous mass on one side of the cytoplasm. 

 The relation of the hyaline plasma-layer to 

 the vitelline membrane and the cytoplasm is 

 brought out very clearly in fertilized eggs 

 which have been plasmolyzed by adding cane 

 sugar to sea water. The protoplasm shrinks 

 and the hyaline layer frequently takes on the 

 appearance of pseudopodial-like processes of 

 the cytoplasm. When the vitelline membrane 

 is dissected from the egg, the hyaline layer 

 remains as an organic part of the cytoplasm. 

 These methods leave no doubt as to the nature 

 of this structure; it is the swollen surface of 

 the cytoplasm and enters into the formation 

 of the larval sea urchin. The water-holding 

 power of the hyaline layer and of the edema- 

 tous vitelline membrane is striking. These 

 structures do not show an appreciable shrink- 

 age in quite concentrated solutions of cane 

 sugar in sea water. 



