30 TEXT-BOOK OF EMBRYOLOGY. 



plane and give rise to the chromosomes of the cleavage nucleus. In the sea- 

 urchin the polar bodies are extruded before the entrance of the spermato- 

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

 entrance of the spermatozoon 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 centrosomes 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 

 spermatozoon. This attraction seems to be of a chemical nature, but is 

 often not limited 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 elements 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 fertilization 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 breeding habits 

 which insure a close proximity of sperms and eggs. 



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

 brane, some (e.g., those of amphibians and of mammals) are permeable to 

 the spermatozoon 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 pro- 

 jection from the surface of the egg, the attraction cone, either precedes or 

 immediately follows the attachment of the spermatozoon to the egg (Fig. 

 15). 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 to the perivitelline space. Should more than one sper- 

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

 the result is an irregular formation of asters and polyasters (Fig. 17), and 

 the early death of the egg either before or soon after a few attempts at 

 cleavage. In some insects, and in selachians, reptiles and birds, a number of 





