34 



PHYSIOLOGICAL TRIGGERS 



Struclure of sea urchin egg surface. Before considering the cortical reaction, 

 the structure of the sea urchin egg surface must be understood. This subject 

 has long been the source of some dispute. The mature, unfertilized egg is en- 

 closed in a layer of gelatinous material which is identical with fertilizin (115, 

 116). Below the jelly the egg is surrounded by a membrane, the viteUine mem- 

 brane (see ref. loi), according to most accounts. Most investigators agree 

 that this membrane is closely applied to the plasma membrane which bounds 

 the egg surface. The cortical region of the egg contains a layer of hyaline ma- 

 terial and the cortical granule layer (36, 37, 50, 74). The cortical granules of 

 the Arbacia egg are approximately 0.8 /x (74) in diameter and form a single 

 layer in the mature egg. A number of workers (27, 74, 76, 97) find that the 

 granules lie in the cortical cytoplasm beneath the plasma membrane (fig. 6A). 

 However, Parpart and Laris (80, 81) conclude from permeability studies that 

 the granules lie between the vitelline membrane and the plasma membrane 

 and, therefore, external to the plasma membrane (fig. 6B). The properties of 



b 



Fig. 6. Structure of the sea urchin egg surface, a, Modified from Runnstrom. b, According 

 to Parpart and Laris. J, egg jelly layer; V, vitelline membrane; C, egg cortex containing the 

 cortical granules; P, plasma membrane; E, endoplasm. 



the isolated egg cortex and the cortex-free egg surface may also be interpreted 

 to support this view (3, 5). Furthermore, this construction is supported by 

 the fact that a single rather than a double membrane appears external to the 

 granules in electron photomicrographs of sectioned eggs (38, 56). However, 

 electron photomicrographs have not yet revealed a membrane (plasma mem- 

 brane of Parpart) internal to the cortical granules. Irrespective of their position 

 in the mature egg, the cortical granules are dispersed throughout the interior 

 cytoplasm of the oocyte. During maturation they move peripherally to occupy 

 their surface position in the mature egg (55, 76, 102). 



The cortical reaction of the sea urchin egg includes the following well estab- 

 lished phenomena: changes in the optical properties of the cortex as seen under 

 dark field illumination (74, 89, 95), a wrinkling of the egg surface (74), break- 

 down of the cortical granules, elevation of the vitelline membrane and its 

 conversion into the fertilization membrane and an increase in the thickness 

 of the hyaline layer. 



