170 



GENERAL EMBRYOLOGY 



abnormal multiple fertilization. In some of the lower plants, 

 after one sperm cell has entered, the egg gives off immediately 

 a chemical substance which actually repels the other sperm 

 congregated about the egg. A frequent method among animals 

 is the secretion of an impenetrable membrane, or a layer of 

 jelly, immediately upon the entrance of one spermatozoon. 

 Or, if a membrane was previously present, its density may be 

 suddenly increased, or an additional membrane formed 



I 



sp f sp 



FIG. 87. Polyspermy in the egg of the Elasmobranch, Torpedo ocellata. 

 From Ziegler, after Ruckert. Germ disc with first cleavage spindle, /, and acces- 

 sory sperm nuclei, sp. 



(Amphioxus), or the micropyle may be closed by the rapid 

 swelling of the egg membranes. Although this process of 

 membrane formation may really have this effect of excluding 

 supernumerary spermatozoa, the general significance of the 

 process renders it doubtful whether this is to be regarded 

 primarily as an event adapted toward this 1 end. 

 ' Occasionally two or more spermatozoa succeed in gaining 

 entrance into the ovum (polyspermy). This ordinarily results 

 in an abnormal course of development, which does not proceed 

 very far before the egg ceases to develop and dies. V A few 

 forms, however, are adapted to the receipt of more than one 

 sperm and polyspermy occurs normally (physiological poly- 

 spermy). Such eggs (Fig. 87) are usually yolk-filled, for 



