264 LIBBIE H. HYMAN. 



original cortex and vitelline membrane and it is definitely known 

 to the observer which is the original and which the new surface 

 of the fragment. In Patiria the whole vitelline membrane clings 

 to the fragment. The fragments show little tendency to assume 

 the spherical form. 



Upon the addition of sperm all such fragments give evidence of 

 the fertilization reaction. The first change noted is an alteration 

 of tension in the fragment. This is evidenced by the disappear- 

 ance of the wrinkles from the vitelline membrane of fragments of 

 urchin eggs and by a change of shape. The fragments tend to 

 assume a more spherical form. This tendency towards sphericity 

 is very marked in the larger fragments as in Figs. 10 and II. 

 From these observations the conclusion can be drawn that the 

 change of tension is confined to the original cortex, since when the 

 amount of cortex remaining is small there is much less change in 

 the shape of the fragment than when it is large. The next change 

 that is noted is the roughening of the surface. This occurs only 

 on that surface of the fragment which was the original surface of the 

 egg. In the smaller fragments the visible fertilization change 

 stops at this point. The membrane follows the crenations of the 

 surface as shown in Figs. 17 and 20; consequently the crenations 

 cannot be due to the formation of small vesicles as supposed by 

 Loeb ('13). In the larger fragments the roughening of the sur- 

 face is followed by the elevation of the fertilization membrane. 

 The membrane elevates only on that surface of the fragment which 

 was the original surface of the egg and which therefore bears a 

 portion of the original vitelline membrane. In the Patiria egg 

 where the whole vitelline membrane remains attached to the 

 fragments, the membrane elevated on the fragment is perfectly 

 continuous with the empty portions of the vitelline membrane 

 as show r n in Figs. 19 to 25. The elevation of the membrane on 

 egg fragments is illustrated in Figs. 10 to 25. 



The fertilization reaction in eggs with extra-ovates is to the 

 same effect. Glaser ('13) records that if the egg of Arbacia is 

 ruptured so that part of the contents flows out, a fertilization 

 membrane appears on one sphere but not on the other. Accord- 

 ing to my observations on the same egg, it is the cortical sphere 

 which elevates a membrane. Chambers ('210, '216) also finds 

 that extra-ovates in the starfish are not fertilizable and do not 



