10 FERTILIZATION 



membrane, as early as 19 19.* Endo (1952) has published some re- 

 markable photographs of cortical granules adhering separately to 

 the inner surface of the fertilization membrane of Clypeaster eggs. 

 The space between the fertilization membrane and the surface of 

 the egg is called the perivitelline space, an unfortunate term as the 

 vitelline membrane is outside this space, not inside it. Globular 

 isotropic cortical granules can sometimes be seen in the perivi- 

 telline space, where they may undergo a spontaneous transforma- 

 tion into positively birefringent rod-shaped particles. If unfertilized 

 eggs are treated with trypsin and then fertilized, these rod-shaped 

 particles are clearly visible. The fusion of the transformed cortical 

 granules with the vitelline membrane is responsible for its harden- 

 ing and transformation into the fertilization membrane, which 

 takes place during the first ten minutes after fertilization. Calcium 

 ions and a third factor which can be extracted from eggs are also 

 concerned in the hardening or 'tanning' of the fertilization mem- 

 brane (Motomura, 1950, 1954; Runnstrom, 195 1). The properties 

 of the fertilization membrane have been studied in great detail 

 under a variety of environmental conditions by Runnstrom and 

 his colleagues. A detailed review of this subject will be found in 

 The Cell Surface in Relation to Fertilization by Runnstrom (1952). 

 One interesting property of the fertilization membrane, which has 

 not received sufficient mention, was described in some detail by 

 Pasteels in 1950. He observed that the fertilization membranes of 

 the eggs of Chaetopterus variopedatus (Renier), Nereis succinea, and 

 of Spisula solidissima (Dillwyn), are contractile. More accurately, 

 at certain times after fertilization the membrane 'expands', thereby 

 becoming creased or folded. The effect is soon reversed and the 

 membrane re-assumes its usual smooth (contracted ?) and spherical 

 appearance. The region on the fertilization membrane where this 

 folding and unfolding phenomenon first occurs, transiently, 20 

 minutes after fertilization in Chaetopterus eggs, is at the vegetative 

 pole, i.e. 180° away from the point of expulsion of the first polar 

 body. The same happens after the expulsion of the second polar 

 body, 30 minutes after fertilization, while 5 minutes later, the 



* Cortical granules and their behaviour at fertilization are much in the lime- 

 light at present; it is therefore only right to mention that some forty-five years 

 ago, E. N. Harvey (191 1, p. 523), said that in the eggs oi Arhacia punctulatn, there 

 were 'numerous minute stained granules, (juite unmoved by the centrifuge. At 

 the time of fertilization these disappear, apparently going to form the substance 

 which passes out of the egg and hardens to a fertilization membrane' ! The stain 

 used was neutral red. 



