194 



Embryogenesis: Preparatory Phases 



S. purpuratus and D. excentricus sperm. In 

 these, interaction can nevertheless be shown 

 by the fact that the sperm of the latter species 

 can absorb S. franciscanus fertilizin. 



It may be concluded, then, that the specific- 

 ity of fertilization is based partly on the 

 specificity of the fertilizin-antifertilizin re- 

 action. Since a number of other interactions 

 are undoubtedly involved in the various steps 

 in fertilization it is not surprising to find 

 that specificity is not entirely determined by 

 one of these. On the basis of the present evi- 

 dence the fertilizin-antifertilizin reaction is 

 concerned in the initial attachment of the 

 spermatozoon to the egg. A scheme for the 

 mechanism of such attachment that has been 

 recently proposed (Tyler, '48a) assumes the 

 same type of interaction that is manifest in 

 antigen-antibody reactions and relates it to 

 the general problem of the mutual adherence 

 of the cells of the various tissues and organs 

 of a multicellular organism. 



Lillie ('19) proposed that fertilizin was 

 also involved in other steps including es- 

 tablishment of the block to polyspermy and 

 activation of the egg through interaction 

 with antifertilizin within the egg. Although 

 the presence of an antifertilizin within the 

 egg has been demonstrated, experimental evi- 

 dence concerning its function is lacking. 



The role of the lytic agents, such as the 

 egg membrane lysin and hyaluronidase, is 

 manifestly to enable the sperm to penetrate 

 the membrane barriers that surround the 

 unfertilized egg. In this action the hyaluron- 

 idase of mammals shows a rather broad 

 species-specificity while that of the egg- 

 membrane lysin of mollusks is relatively nar- 

 row. No evidence is as yet available to in- 

 dicate whether or not these agents are 

 involved in the further penetration of the 

 surface of the egg proper. For the egg-surface 

 lysin Runnstrom ('49) suggests a role in the 

 establishment of the block to polyspermy. 

 While this is in harmony with its fertiliza- 

 tion inhibiting properties, the availability 

 of this agent under physiological conditions 

 needs to be demonstrated. 



The work on hyaluronidase stimulated 

 many attempts to apply this agent clinically 

 to overcome sterility in hvimans. Although 

 there were some early claims of success, 

 recent controlled experiments (Chang, '47; 

 Leonard et al., '47) with rabbits and rats 

 have shown that the addition of hyaluron- 

 idase to inseminates does not enhance fer- 

 tilization and that fertilization can be 

 effected without visible dispersal of the fol- 

 licle cells. These results do not refute the 



above assigned role but rather show that 

 the individual spermatozoon carries sufficient 

 enzyme to make a path for itself through the 

 follicle coat of the egg. 



REACTION OF THE EGG 



Cortical Change and Block to Polyspermy. 



The visible changes that occur at the surface 

 of the egg have been investigated extensively, 

 especially in sea urchins, and most of the 

 recent work is discussed in the review by 

 Runnstrom ('49). Mention may be made here 

 of newer work by Rothschild and Swann 

 ('49), who have examined the question of 

 whether or not the rate of propagation of 

 the visible cortical change is sufficient to 

 account for the block to polyspermy. These 

 workers followed cinematographically the 

 darkfield brightening of the surface of the 

 egg that spreads out from the point of 

 contact of the spermatozoon and constitutes 

 the first visible cortical change. They find 

 that the time required for this change to 

 progress to the opposite side of the egg 

 averages 20 seconds at 18° C. in Echinus 

 esculentus. To decide whether or not this is 

 rapid enough to account for the block to 

 polyspermy, estimates are made of the 

 chance of a second sperm striking an un- 

 altered part of the surface. For these calcula- 

 tions measurements are made of the trans- 

 latory speed of swimming of the spermatozoa. 

 The frequency of collision with an egg is 

 then calculated from the kinetic equation, 

 N—TTcP'nc, in which a is the radius of the egg 

 (50 microns), n is the density of sperm sus- 

 pension and c is the mean translatory speed 

 of the spermatozoa (200 microns per second). 

 For sperm densities of 10^, 10^ and 10''' per 

 milliliter the number of collisions per second 

 would be 0.16, 1.6 and 16, respectively. Since 

 a density of 10'^ per milliliter does not give 

 any appreciable polyspermy in sea urchin 

 eggs that are in good condition it is concluded 

 either that the block to polyspermy is es- 

 tablished much more rapidly than the ob- 

 served surface change or that only a small 

 fraction (about ^Aeo) of collisions result in 

 fertilization. Further experiments with 

 oocytes, that respond to insemination by for- 

 mation of papillae, each of which is associ- 

 ated with a spermatozoon, show the number 

 of papillae to be considerably less than the 

 number of sperm-oocyte collisions. On the 

 basis of this and other considerations, Roths- 

 child and Swann favor the view that the ob- 

 served cortical change may represent the 

 block to polyspermy and that many of the 



