36 PHYSIOLOGICAL TRIGGERS 



granule breakdown. Allen further showed that the effect was specific when 

 reciprocal crosses were attempted between two species of sea urchins. This 

 result strengthens the view that the sperm activated the isolated cortex by the 

 normal fertilization mechanism. However, since the external surfaces of the 

 isolated fragments were attached to glass in many cases, it is not immediately 

 clear how the activating sperm reached the normal site of activation. The 

 experiment raises the interesting possibility that the sperm can activate by 

 contact with either side of the cortex. Since fertilization membrane formation 

 and cortical granule breakdown result from insemination of the cortical frag- 

 ments, the normal propagative response probably operates in these fragments. 

 This result would indicate that the cortex alone can propagate the response. 

 However, it may be argued that a small amount of endoplasm adheres to the 

 isolated cortex. The force of this argument will depend to a considerable extent 

 on the interpretation given to the structure of the egg surface (see earlier sec- 

 tion). 



The evidence for a cortical transmission of the response advanced by Runn- 

 strom and Kriszat (100) has been challenged by Rothschild (88). The former 

 investigators found that areas of jellyless eggs adhering to glass did not undergo 

 the cortical reaction. They reasoned that the adhering region is altered suffi- 

 ciently to prevent propagation in the cortex. However, as Rothschild (88) 

 points out, it may be argued with equal force that the alteration may prevent 

 a response of the cortex to an endoplasmically propagated stimulus. 



In further support of the endoplasmic transmission hypothesis, Rothschild 

 (88) cites experiments of Horsladius and Runnstrom (41). These workers 

 inseminated eggs constricted in capillary tubes. The eggs frequently elevated 

 membranes only on the end opjwsite that of insemination. This result is most 

 readily explained by assuming that an endoplasmic stimulus reaches the egg 

 surface only at the far end of the stretched egg. However, this interpretation 

 must be treated with caution, for the physical properties of the two ends of 

 eggs constricted in capillaries are evidently difTerent (26, 41). These physical 

 differences might well reflect differences insusceptibility to a propagated cortical 

 response. Furthermore, the cortex of constricted eggs showed dark field cortical 

 changes in the elongated area following insemination, even though membranes 

 did not form in this region. 



Mechanism of propagalioii. The nature of the transmission mechanism of 

 the i)ropagated cortical response is even more obscure than the region of the 

 egg through which it passes. In accord with many earlier workers, Sugiyama 

 (no) believes that an invisible cortical reaction precedes the first visible evi- 

 dence of cortical change. Sugiyama's view is based on the observation that 

 certain agents induce local, non-propagative cortical granule breakdown. 

 Accordingly, Sugiyama reasons that breakdown of one cortical granule does 



