3© PHYSIOLOGICAL TRIGGERS 



to the activation initiating mechanism directly will be considered here. These 

 are the reversal of fertilization and variations in the normal fertilization re- 

 sponse that may be attributed to the condition of the gametes. 



Reversibility of fertilization. Complete reversal of fertilization might be con- 

 sidered to require expulsion of the fertilizing sperm from the egg cytoplasm, 

 reversal of meiosis in some forms, and return to the morphological and physio- 

 logical conditions of the unfertilized egg. Such reversal has not been obtained, 

 but an approximation of it has been described by Tyler and Schultz (129) 

 for early stages in the fertilization of the echiuroid, Urechis. 



The unfertilized egg of Urechis is indented. Upon insemination the activating 

 spermatozoan attaches to the egg surface. Three minutes later the indentation 

 of the egg disappears. Subsequently, the spermatozoan enters the egg and the 

 fertilization membrane forms. If the egg is transferred to acidified sea water 

 within 3 minutes following insemination, the egg rounds out and the sperm 

 enters as in normal fertilization. However, in the acid sea water further de- 

 velopment ceases, the indentation of the egg reappears and the egg returns to 

 the prefertilization condition. If such an egg is transferred to normal sea water, 

 development is not resumed and the sperm remains passively in the egg cyto- 

 plasm. If, now, the egg is reinseminated, a second sperm attaches to the egg, 

 the egg again rounds out, the second sperm enters and development proceeds, 

 but the development is now dispermic for both the first and the second sperms 

 contribute to the division figure. Trispermic development may be obtained if 

 the egg is given a second acid treatment and a third insemination. 



Evidently the development of the Urechis egg is not only blocked by the 

 acid treatment but certain of the initial activation changes are actually re- 

 versed. Notable among these are the rounding out of the egg and the block 

 to polyspermy. Blocked and reversed eggs can be reactivated by treatment 

 with an appropriate parthenogenetic agent (121, 129) as well as by a second 

 insemination. Attempts to duplicate this reversal of fertilization in the sea 

 urchin have failed (91). 



Unfortunately this remarkable experiment has so far served mainly to demon- 

 strate reversibility of activation. A further examination of the reversed eggs, 

 employing methods recently developed for the sea urchin, might now be in 

 order. One point of {)articular interest would be an attempt to extend the 

 period between the initial insemination and the acid treatment. Possibly pre- 

 treatment with trypsin or other agents which prevent elevation of the fertiliza- 

 tion membrane in the sea urchin would be effective. 



Effects resulting from the condition of the gametes. It has long been known (49) 

 that even under favorable conditions insemination does not always result in 

 optimal fertilization and development of the egg. Variations from the accepted 

 normal are usually ascribed to a suboptimal condition of the gametes, especially 

 the egg. Deviations from the normal pattern include partial or complete failure 



