148 FERTILIZATIOX OF THE OVUM 



fourth cleavage. Such embryos develop as far as the blastula stage, 

 but never form a gastrula.^ In cases where several spermatozoa 

 normally enter the *^'g^^ (physiological polyspermy), only one of the 

 sperm-nuclei normally unites with the egg-nucleus, the supernumerary 

 sperm-nuclei either degenerating, or in rare cases — e.g. in elasmo- 

 branchs and reptiles — living for a time and even dividing to form 

 " merocytes " or accessory nuclei. The fate of the latter is still in 

 doubt ; but they certainly take no part in fertilization. 



It is an interesting question how the entrance of supernumerary 

 spermatozoa is prevented in normal monospermic fertilization. In 

 the case of echinoderm-eggs Fol advanced the view that this is 

 mechanically effected by means of the vitelline membrane formed 

 instantly after the first spermatozoon touches the Q^^. This is indi- 

 cated by the following facts. Immature eggs, before the formation 

 of the polar bodies, have no power to form a vitelline membrane, 

 and the spermatozoa always enter them in considerable numbers. 

 Polyspermy also takes place, as O. and R. Hertwig's beautiful ex- 

 periments showed ('87), in ripe eggs whose vitality has been dimin- 

 ished by the action of dilute poisons, such as nicotine, strychnine, 

 and morphine, or by subjection to an abnormally high temperature 

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

 formed, so that several spermatozoa have time to enter.^ 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 ; and 

 Calberla suggested {Pctnv/iyrjon) 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.'^ 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. 



^ For an account of the internal changes, see p. 261. 



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

 Normally requiring the stimulus of only a single spermatozoon f(jr the formation of the vitel- 

 line meml:>rane, it here demands the more intense stimulus of two, three, or more before 

 the membrane is formed. Tliat the membrane is not present before fertilization is admitted 

 by Hertwig on the ground stated at p. 97. 



^ 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. 



