33O ROBERT CHAMBERS. 



to bring about fertilization. When a cloud of sperm has been 

 blown upon a naked egg, one may frequently observe a sperma- 

 tozoon swim toward it, wander over its surface, and then swim 

 away. On the other hand, the empty membrane with its in- 

 vesting jelly immediately becomes covered with a halo of sper- 

 matozoa. This observation accords with the interpretation of 

 Buller ('02), that the investing jelly determines the direction of 

 the sperm which are captured by it, and that there is no apparent 

 chemotaotic substance excreted by the egg to attract the sperm. 



The difference in position of the polar bodies in the starfish egg 

 with respect to the fertilization membrane as shown by Gemmill 

 ('12) (see also Chambers and Mossop, '18, and Carrey, '19) may 

 be explained as follows : When the polar bodies form prior to fer- 

 tilization they rise off the surface of the egg, carrying with them 

 the closely adherent membrane. When they are pinched off the 

 egg membrane remains continuous about the egg and subsequent 

 insemination results in the formation of a fertilization membrane 

 with the polar bodies lying outside. If, however, the eggs are in- 

 seminated before extrusion of the polar bodies, the egg membrane 

 lifts off as the fertilization membrane and, when the polar bodies 

 are formed, they lie within the membrane. 



In the sea-urchin egg the identity of the egg membrane with the 

 fertilization membrane is more difficult to demonstrate. In Fig. 

 14 is shown the effect of locally injuring the surface of the sea- 

 urchin egg. In a is a disintegrated mass produced by tearing a 

 spot on the surface with a needle. In b this area is shown as a 

 bulge which may be explained as being produced by the interior 

 pressure of the egg on a surface weakened by the loss of an invest- 

 ing membrane. In c the egg has been fertilized. The fertiliza- 

 tion membrane is formed over all the surface except at the in- 

 jured place. In d segmentation has occurred and a blastomere 

 protrudes through the gap in the fertilization membrane. 



A better demonstration is the case shown in Fig. 15. At 4:26 

 the tip of a needle was punched through the cortex. Within a few 

 seconds the cytoplasm distal to the needle flowed away, leaving the 

 needle lying under a delicate membrane (Fig. 15-0). At 4:27 

 the egg was inseminated with the needle still in place. At 4 : 29 



