CHARLES li. METZ 4 1 



nicotine, the mechanism of action of the agents has not been examined and is 

 unknown. Nicotine does not increase the speed of the sperm, the rate of propa- 

 gation of the dark field change in the egg cortex, or the time of appearence of 

 the fertiUzation membrane {;};^, 87, 90). Therefore, the action of nicotine is 

 probably not upon the sperm or the slow speed block to polyspermy. Likewise, 

 nicotine does not increase the receptivity of the egg surface sufficiently to 

 account for the facts of polyspermy. Therefore Rothschild (87) concludes 

 that nicotine induces polyspermy by abolishing the high speed block to poly- 

 spermy. 



The studies of Sugiyama (109) and Hagstrom and Hagstrom (35) have shown 

 that fertilized sea urchin eggs can be 'refertilized' following appropriate treat- 

 ment. In these experiments the fertilization membranes were removed or 

 their formation prevented by mechanical or chemical (trypsin, chymotrypsin, 

 urea) treatment. Subsequently the membraneless, fertilized eggs were treated 

 with Ca- and/or Mg-free sea water or urea, returned to sea water and rein- 

 seminated. In some of the experiments over 90 per cent of such reinseminated 

 eggs became polyspermic. According to these investigators even the blastomeres 

 of early cleavage stages can be refertilized. These studies have been confirmed 

 and extended by Tyler, Monroy and Metz (126) in Lytechiuiis pictus and L. 

 variegalus. The eggs of these sea urchins will refertilize following simple mechani- 

 cal removal of the fertilization membrane. The membranes were removed by 

 squirting the eggs through a syringe 1-2 minutes after the initial insemination. 

 Following reinsemination, 100 per cent polyspermy was obtained in some 

 tests. Refertilization of the membraneless eggs was obtained \\\) to 40 minutes 

 after the first insemination. From these results it is clear that no absolute 

 cytoplasmic block to polyspermy develops within the first 40 minutes following 

 fertilization in the Lytechinus egg. In Lytechinus, and probably in other sea 

 urchins as well, the fertilization membrane apparently constitutes the final, 

 low speed block. In the species examined by Sugiyama and by Hagstrom and 

 Hagstrom the hyaline plasma layer may also constitute a block to polyspermy. 

 This layer is removed by the urea or Ca-Mg-free sea water treatment. Since 

 this treatment appears to be necessary for refertilization in these forms, it 

 is assumed that removal of the hyaline layer is required for refertilization. 



In most of the experiments cited, the high speed, partial block (92) should 

 have developed before treatment of the eggs. Evidently, then, the eggs re- 

 fertilize in spite of any such partial block, or the block is not effective under 

 the conditions of the experiments. This interesting point will require further 

 study. 



CONCLUSIONS 



The orderly nature of the early events of development suggests that these 

 proceed through a branching system of 'chain reactions' from a few or possibly 

 a single reaction between the interacting cells. This primary activating re- 



