FERTILIZATION REACTION IN ECHINODERM EGGS. 259 



fullest extent whereas in the starfish egg the membrane separates 

 from the entire surface of the egg before it elevates to any con- 

 siderable extent (Fig. 9). Consequently in the urchins the egg 

 at first lies very asymmetrically placed within 

 the membrane (Figs. 4 and 5). This asym- 

 metry is further emphasized by the flattening of 

 the egg at the region of initial elevation. Later 

 symmetry is restored in normal eggs by the wide- 

 ning of the perivitelline space at other regions 

 and by the resumption of spherical form by the FlG 9 stage in 

 egg. In subnormal eggs the asymmetry is likely the fertilization re- 

 to persist. In the Patiria egg, the membrane sep- actlon of Patma 



e r r i -11 ntiniata. The vit- 



arates from the whole surface of the egg with the dline membrane 

 formation of only a very narrow perivitelline separates only 

 space; later this widens simultaneously around slightly from the 



'-ni a j_i r -j.- i e gg a t first. Note 



the egg. The flattening at the region of initial 



also the crenation 



membrane elevation is slight in the starfish egg O f tne egg surface. 

 but generally perceptible. 



The separation of the vitelline membrane does not proceed 

 with entire smoothness from the point of initiation but the mem- 

 brane tends to adhere to the egg surface at some points slightly 

 longer than at others. An exaggeration of this tendency results in 

 the vesicle formation described and figured by Loeb ('13), ob- 

 servable according to him through retarding the normal reaction 

 by lowering the temperature. Vesicle formation also occurs at 

 ordinary temperatures in subnormal eggs and in such eggs the 

 vesicles may persist, not flowing together to form a continuous 

 perivitelline space. Pronounced vesicle formation seems to be 

 due to a subnormal response of the egg to fertilization. 



An attraction cone at the site of sperm entry such as has been 

 reported by others was not noticed but was not particularly 

 sought for. I was also unable to observe the passage of materials 

 from the cortex into the perivitelline space, as described by Just 

 ('19) for Echinarachnins, although I searched carefully for such 

 a process. 1 



1 In making this statement I do not intend to imply the slightest question of the 

 validity of Just's observations. I believe from other lines of evidence that material 

 does pass from the egg into the perivitelline space but the process is invisible in the 

 eggs with which I have worked. 



