200 FERTILIZATION OF THE OVUM 



(31 C); and in these cases the vitelline membrane is only slowly 

 formed, so that several spermatozoa have time to enter. 1 Similar 

 mechanical explanations have been given in various other cases. 

 Thus Hoffman believes that in teleosts the micropyle is blocked by 

 the polar bodies after the entrance of the first spermatozoon ; arri 

 Calberla suggested (Petromyzoii) that the same result might be 

 caused by the tail of the entering spermatozoon. It is, however, 

 far from certain whether such rude mechanical explanations are 

 adequate ; and there is considerable reason to believe that the egg 

 may possess a physiological power of exclusion called forth by the 

 first spermatozoon. Thus Driesch found that spermatozoa did not 

 enter fertilized sea-urchin eggs from which the membranes had been 

 removed by shaking. 2 In some cases no membrane is formed (some 

 coelenterates), in others several spermatozoa are found inside the 

 membrane (nemertines), in others the spermatozoon may penetrate 

 the membrane at any point (mammals), yet monospermy is the rule. 



I. Immediate Results of Union 



The union of the germ-cells calls forth profound changes in both. 



(a) The Spermatozoon. Almost immediately after contact the tail 

 ceases 'its movements. In some cases the tail is left outside, being 

 carried away on the outer side of the vitelline membrane, and only 

 the head and middle-piece enter the egg (echinoderms, Fig. 100). 

 In other cases the entire spermatozoon enters (amphibia, earthworm, 

 insects, etc., Fig. 89), but the tail always degenerates within the 

 ovum and takes no part in fertilization. Within the ovum the 

 sperm-nucleus rapidly grows, and both its structure and staining- 

 capacity rapidly change (cf. p. 182). The most important and signifi- 

 cant result, however, is an immediate resumption by the sperm-nucleus 

 and sperm-centrosome of the power of division, which has hitherto 

 been suspended. This is not due to the union of the germ-nuclei ; 

 for, as the Hertwigs and others have shown, the supernumerary 

 sperm-nuclei in .polyspermic eggs may divide freely without copu- 

 lation with the egg-nucleus, and they divide as freely after entering 

 enucleated egg-fragments. The stimulus to division must therefore 

 be given by the egg-cytoplasm. It is a very interesting fact that 

 in some cases the cytoplasm has this effect on the sperm-nucleus 



1 The Hertwigs attribute this to a diminished irritability on the part of the egg-substance. 

 Normally requiring the stimulus of only a single spermatozoon for the formation of the vitel- 

 line membrane, it here demands the more intense stimulus of two, three, or more before the 

 membrane is formed. That the membrane is not present before fertilization is admitted by 

 Hertwig on the ground stated at page 132. 



2 On the other hand, Morgan states ('95, 5, p. 270) that one or more spermatozoa will 

 enter nucleated or enucleated egg-fragments whether obtained before or after fertilization. 



