MICRODISSECTION STUDIES. 



329 



face of such eggs with the needle, the membrane is often torn 

 in such way that the egg slips out leaving the membrane stuck 

 to the needle. Such an egg, when inseminated, is fertilized and 

 subsequently segments with no investing membrane whatever. 



FIG. ii. a, starfish egg cut in two without destroying the investing mem- 

 brane, b, after insemination the investing membrane lifts off both fragments 

 as the fertilization membrane, c, one of the fragments segmented, the other 

 did not. That both fragments lie in a common cavity is shown by the en- 

 croaching of blastomeres of one fragment into the region of the unsegmented 

 fragment. 



FIG. 12. a, starfish egg cut into three pieces. One piece was squashed and 

 produced an exovate. b, on being fertilized the exovate was pinched off 

 as an endoplasmic sphere (cf. Fig. 25). The rest of the fragments produced 

 a common fertilization membrane. Each of the three enclosed fragments 

 developed into a swimming larva. 



FIG. 13. a, sand-dollar egg rolled as it was cut in two. The egg membrane 

 between the two pieces was twisted into a thread joining the two. b, egg 

 shortly after fertilization showing fertilization membrane about each con- 

 nected by a filament, c, the two pieces in an early segmentation stage. 



The difference in reaction of sperm to an egg which has been 

 denuded of its membrane as well as of its jelly, and to one which 

 has not is very striking. An egg within its membrane is quickly 

 surrounded by spermatozoa as they are trapped in the jelly sur- 

 rounding the membrane. In a membraneless egg no crowding 

 of spermatozoa is noticeable and heavy insemination is necessary 



