THE MATURE EGG, UNFERTILIZED AND FERTILIZED 83 



pyle in their studies on the localization of materials in the Arbacia egg. 

 Though the micropyle functions as the place of entry of the sperm 

 in some forms, this is not true for sea urchins, for the sperm can enter 

 at any point. This can readily be determined in centrifuged, stratified 

 eggs. 



j. Membranes and Layers. Figures 9 and 10 



I. Unfertilized Egg 



Egg diameter^ without membranes, 74 \l. 



Jelly layer. On outside of egg. 28 to 32 \x thick (E. B. H.). Plate XVI, 

 Photograph 3. 



Vitelline membrane. Outside of plasma membrane; difficult to distin- 

 guish from plasma membrane. Lifts off on fertilization to form fertili- 

 zation membrane. Not measurable with light microscope; 25 m/^ with 

 electron microscope (E. B. Harvey and Anderson, 1943). 



Plasma membrane. Lies over cortical layer. 10 mu, (Danielli, 1942, 

 p. 72). Located inside cortical layer according to Parpart and Laris 



(1954)- 



Cortical layer. Location of cortical granules which disappear on fertili- 

 zation. Thickness of layer in A. punctulata is 0.8 [jl; i to 2 fx in other 

 eggs (Monroy and Oddo, 1946; Runnstrom, 1949b; Mitchison, 1952). 



2. Fertilized Egg 



Jelly layer. As in unfertilized egg. 



Fertilization membrane. 25 m\x when first formed from vitelline mem- 

 brane; becomes thicker and tougher (E. B. Harvey and Anderson, 

 1943). Less than .03 \i with electron microscope (Hillier, Lansing, 

 and Rosenthal, 1952). 



Perivitelline space. Between fertilization membrane and hyaline layer 

 3 to 5 IX (E. B. H.). 



Hyaline layer (or ectoplasmic layer). An investing layer which binds 

 the blastomeres together, 2 to 3 [x thick when fully formed, 15 to 20 

 minutes after fertiHzation (E. B. H.). 



Plasma membrane. As in unfertilized egg. 



Cortical layer. As in unfertilized egg but no cortical granules. Most of 

 pigment granules located here after fertiHzation (McClendon, 1909b 

 et al.). 



Data on the various layers and membranes will be found under the 

 appropriate heading in Part IV. 



