36 TEXT-BOOK OF EMBRYOLOGY. 



sperm nucleus now approach the female pronucleus, the aster leading and its 

 rays rapidly extending. On or before reaching the female pronucleus the aster 

 divides into two daughter asters (Fig. 23, b) which separate with the formation 

 of the usual central spindle, while the two pronuclei unite in the equatorial 

 plane and give rise to the chromosomes of the cleavage nucleus (Fig. 23, c and 

 d). In the sea-urchin the polar bodies are extruded before the entrance of the 

 spermatozoon. In cases where the polar bodies are not extruded until after 

 the entrance of the spermatozoon (Ascaris, Fig. 14) the amphiaster forms while 

 waiting for their extrusion, the nuclei joining subsequently. When the sperm 

 head finds the polar bodies already extruded, union of the two pronuclei may 

 take place first, followed by division of the centrosome and the formation of the 

 amphiaster. 



The coming together of ovum and spermatozoon is apparently determined 

 in some cases by a definite attraction on the part of the ovum toward the sperma- 

 tozoon. This attraction seems to be of a chemical nature, but is often not lim- 

 ited to the attraction of spermatozoa of the same species. Foreign spermatozoa 

 will be attracted and will enter the ovum if they are physically able to do so. 

 The entrance of these spermatozoa may even start the process of cleavage, 

 though such cleavage is usually abnormal and does not progress very far. That 

 this attraction is not dependent upon the integrity of the ovum as an organism 

 is shown by the fact that small pieces of egg cytoplasm free from nuclear ele- 

 ments exert the same attractive force, so that spermatozoa are not only attracted 

 to them, but will actually enter them. In other cases the stimulus for fertiliza- 

 tion is obviously one of contact. The spermatozoa of some Fishes will swim 

 around at random until they touch any object when they become attached and 

 are unable to escape. Fertilization in these cases is therefore a matter of chance 

 favored by the enormous number of sperms produced, and by the special breed- 

 ing habits which insure a close proximity of sperms and eggs. 



Of eggs which are enclosed by a distinct membrane, the vitelline membrane, 

 some (e.g., those of Amphibians and of Mammals) are permeable to the sper- 

 matozoon at all points; others have a definite point at which the spermatozoon 

 must enter, this being of the nature of a channel through the membrane the 

 micropyle. In some instances a little cone-shaped projection from the surface 

 of the egg, the attraction cone (Fig. 22, i), either precedes or immediately fol- 

 lows the attachment of the spermatozoon to the egg. Instead of a projection 

 there may be a depression at the point of entrance. 



There seems to be no question that but one spermatozoon has to do with 

 the fertilization of a particular ovum. In Mammals only one spermatozoon 

 normally pierces the vitelline membrane although several may penetrate the 

 zona pellucida (Fig. 22, i) to the peri vitelline space. Should more than one 

 spermatozoon enter such an egg as, for example, in pathological polyspermy 



