CHARLKS B. METZ 35 



These cortical changes proceed across the egg surface in wave-Hke fashion 

 from the point of sperm attachment. The time required for the most rapid 

 change, namely the dark tield change, is 20 seconds at i8°C" in the material of 

 Rothschild and Swann (8g). By direct observation, Allen and Hagstrom (5) 

 lind both the dark lield change and granule breakdown to occur simultaneously. 

 The wrinkling of the egg surface is also temporally related to cortical granule 

 breakdown (74). Elevation of the vitelline membrane and the increase in the 

 hyaline layer follow some seconds later. 



This series of activation phenomena and the wave-like manner of their 

 passage over the egg surface raise several interesting questions. These include 

 the following: Are the various features of the cortical response interdependent 

 or can they occur independently? Does the propagative mechanism for these 

 responses pass through the cortex, the endoplasm or both parts of the egg? 

 What is the mechanism of propagation? Answers to these questions will be 

 sought in the paragraphs below. 



Interrelation of visible cortical responses. Several of the visible cortical events 

 can occur independently of one or more of the others. In fact, it is not unlikely 

 that the egg can at least cleave in the absence of any of the visible changes. 

 Thus eggs inseminated following butyric acid treatment can cleave, although 

 they fail to elevate membranes or undergo granule breakdown (76). However, 

 formation of a substantial hyaline layer is evidently necessary to cement the 

 blastomeres together. As is well known, the blastomeres separate and normal 

 development fails in the absence of the hyaline layer. Substantial elevation of 

 the membrane depends upon discharge of the cortical granules (81). Likewise, 

 hardening of this membrane to form the fertilization membrane depends upon 

 incorporation of cortical granule material into the vitelline membrane (27, 

 76, 97). The marked birefringence of the fully formed fertilization membrane 

 also depends upon this incorporation of cortical granule material (42, 97). The 

 dark tield color change of the egg cortex and granule breakdown can occur 

 independently in suboptimal material (5, 97) or following treatment with 

 antifertilizin or basic proteins (97). However, the dark field changes, cortical 

 wrinkling and hyaline layer thickening do appear to be closely related to granule 

 breakdown for these responses occur together in graded fashion and in pro- 

 portion to the degree of stimulation in eggs treated with an alternating cur- 

 rent (6). 



Cortical vs. endoplasmic propagation. The wave-like passage of the visible 

 responses over the egg surface suggests that the response is propagated in the 

 egg cortex. Runnstrom and Kriszat (100), Allen (2) and Allen and Hagstrom 

 (5) subscribe to this view. Rothschild (84, 88) suggests from a consideration of 

 the rate of propagation that the impulse may pass through the endoplasm of 

 the egg. In support of the first view, Allen (3) showed that fragments of the 

 egg cortex could be isolated, and that such fragments would respond to in- 

 semination by formation of a thin fertilization membrane and by cortical 



