42 PHYSIOLOGICAL TRIGGERS 



action of fertilization would appear to be a chemical reaction between the 

 interacting cells. The high order specificity of fertilization may reside in the 

 structure of the substances that interact in this reaction. 



These views have been entertained for many years, but until recently no 

 substantial, positive experimental evidence has been produced to support them. 

 Many 'sex substances' have been found in the most diverse plant and animal 

 groups and a number of these have been shown to play important or essential 

 roles in fertilization, but in no single instance has a speciiic organic substance 

 been isolated, characterized and demonstrated to participate directly in the 

 activation initiating reaction of fertilization. The closest approximation to 

 such concrete evidence is found in the studies on Paramecium. 



In Paramecium, highly specific surface proteins, the mating type substances, 

 interact in antigen-antibody-like fashion to initiate the morphological, physio- 

 logical and cytological changes of fertilization. Evidence for this role of the 

 mating type substances has been obtained primarily from studies on the specific 

 activation of living paramecia by dead animals of opposite mating type. As 

 yet the mating type substances have not been extracted from living or dead 

 animals in active form. 



Among Afetazoa positive evidence for a triggering action at fertilization is 

 wanting. However, the formation of an acrosome filament following treatment 

 of sperm w'ith egg water may prove to be a trigger-like response to a specific 

 agent from eggs. Although the activation of the egg frequently appears to 

 show all-or-none behavior, graded responses at fertilization have long been 

 known. In the most thoroughly analyzed cases the suboptimal quality of the 

 fertilization can be attributed to the condition of the sperm. IMetal ions seem 

 to be involved in this effect. The fact that the degree of response of the egg can 

 be determined by the condition of the sperm argues strongly against a simple, 

 all-or-none trigger mechanism. Further examination of these graded responses, 

 and a serious effort to relate them to the graded responses of the egg following 

 parthenogenetic activation, should contribute substantially to an understand- 

 ing of the activation initiating mechanism of the egg. 



Following the activating reaction a series of changes occurs in both the 

 cortex and endoplasm of the egg. These changes are clearly propagated from 

 the site of sperm union, but the nature of the primary propagative reaction 

 and the region or regions of the egg through which it passes have not yet been 

 clearly established. One promising approach would appear to result from the 

 discovery of a resting potential across the egg surface and the potential change 

 associated with the activation of the egg. Further studies may shortly relate 

 the propagative system of the egg to that of muscle and nerve. 



REFERENCES 



1. Afzelius, B. a. Z. Zellforsch. u. mikroscop. Anal. 42: 13^, 1955. 



2. Allen, R. D. Expll. Csll Research b: 412, 195 ). 



