FERTILIZATION. 37 



exists. The middle piece of the spermatozoon always enters the ovum with the 

 head. It has already been shown (p. 24) that one or two spermatid centro- 

 somes take part in the formation of the middle piece. Male centrosome 

 elements are therefore undoubtedly carried into the ovum in the middle piece. It 

 is equally well known, for some forms at least, that the centrosome of the ovum 

 disappears just after the extrusion of the second polar body. In a considerable 

 number of forms the development of the centrosome of the fertilized egg from, or 

 in close relation to the middle piece of the spermatozoon has been observed. 

 The details of the process as it occurs in the sea-urchin have been care- 

 fully described by Wilson. In cases of this type the tail of the spermato- 

 zoon remains outside the egg while the head and middle piece, almost im- 

 mediately after entering, turn completely around so that the head points away 

 from the female pronucleus (Fig. 23, a). An aster with its centrosomes next 

 appears, developing from, or in very close relation to the middle piece. The 

 aster and 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 ex- 

 truded until after the entrance of the spermatozoon (Ascaris, Fig. 9) the 

 amphiaster forms while waiting for their extrusion, the nuclei joining sub- 

 sequently. 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 

 largely by a definite attraction on the part of the ovum toward the spermato- 

 zoon. This attraction seems to be chemical in nature and is specific for germ 

 cells of a particular species, that is, ova possess attractive powers toward 

 spermatozoa of the same species only. This has been proved in some of the 

 lower forms by mixing ova and spermatozoa in a suitable medium with the re- 

 sult that the spermatozoa become attached to the membrane of the ova in large 

 numbers. Spermatozoa of other species will not, however, be thus attracted. 

 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. 



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 

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



