FERTILIZATION REACTION IN ECHINODERM EGGS. 257 



the cytoplasm of Tripneustes and Echinarachnius eggs as of about 

 the consistency of glycerine. The cytoplasm of the eggs which I 

 have investigated seems to me to correspond to 4 or 5 in Seifriz's 

 scale ('20, p. 364). The existence of a membrane around the 

 unfertilized egg has also been asserted by many other investiga- 

 tors apart from considerations of viscosity differences. 



The vitelline membrane is probably not sharply delineated 

 from the less viscous egg cytoplasm. According to Chambers 

 ('lya) the external jellied surface of the egg cell "gradually passes 

 into the sol of the interior." This gradation must be very abrupt 

 as I did not notice it in my experiments although I think the 

 superficial cytoplasm does undoubtedly adhere to the vitelline 

 membrane. 



j. The Morphology of the Fertilization Process. The fertiliza- 

 tion reaction has been observed on eggs mounted in plenty of 

 water without a cover glass; on eggs in a depression slide with a 

 cover glass; and on eggs in a hanging drop on the under surface 

 of a cover glass placed over a depression. The reaction has been 

 studied with low, medium, and high powers. The phenomena 

 observed were the same by any method of mounting the eggs 

 and cannot possibly be ascribed to compression, for the eggs were 

 never under compression. 



The fertilization reaction begins to be visible 45 to 60 seconds 

 after mixing the eggs w T ith dilute sperm suspension. It is first 

 indicated by a roughening or crenation of the egg at one place 

 on the surface and this roughening spreads very rapidly in all 

 directions from the initial place over the entire surface of the 

 egg. Following closely upon this change the vitelline membrane 

 begins to elevate. This elevation starts at the same place as the 

 roughening and like the latter sweeps in all directions over the 

 egg. The roughening and the elevation of the membrane occur 

 at such a close interval as to be almost simultaneous but it is 

 certain that the former precedes. There is also to be noted a 

 flattening of the egg at the region of initial membrane elevation. 

 This flattening is commonly very marked at the site of initial 

 membrane elevation and spreads from this for a short distance 

 but never extends more than halfway over the egg. The fertiliza- 

 tion changes in S. franciscanus are illustrated in Figs. I to 5. 



The time required for these changes to pass over the egg is 



