192 FERTILIZATIOX OF THE OVUM 



rays do not centre in the nucleus, as Hertwig described, /'/// at a 

 point in advance of it — a fact afterward confirmed by Hertwig 

 himself and by Boveri ('88, i ). Hertwig and Fol afterward found 

 that in cases of po]\-spermy, when several spermatozoa enter the egg, 

 each sperm-nucleus is accompanied by an aster, and Hertwig proved 

 that each of these might give rise to an amphiaster (Fig. loi). In 

 1886-87 Vejdovsky brought forward strong evidence to show that 

 in the fresh-water annelid RhyncJichnis the cleavage-amphiaster 

 arises directly from the sperm-amphiaster, itself derived by the 

 division of a " periplast " (attraction-sphere) imported into the ^g^ by 

 the spermatozoon, while the polar amphiaster entirely disappears. 

 It was Boveri i^'i'J, 2) who first carefully studied the facts with 

 reference to the centrosome, reaching the conclusion (in the case of 

 Ascaris and the sea-urchin) that a single centrosome is brought 

 in by the spermatozoon, and that it divides to form two centres about 

 which are developed the two asters of the cleavage-figure. He was 

 thus led to the following conclusion, which has received the sup- 

 port of man)' later in\'estigators : T/ic ripe egg possesses all of the 

 0)gans and qualities necessary for division excepting the centrosome, 

 by which division is initiated. The spcrviatozo'dn, on the other hand, 

 is provided zvith a centrosome, but hicks the substance in which this 

 organ of division may exert its activity. TJirougJi the union of the 

 two cells in fertilization, all of the essential organs necessary for 

 division are brought togetlier ; the egg nozv contains a centrosome 

 zvhicJi by its oivn division leads the way in the embryonic develop- 

 vient} Very numerous observations, supporting this conclusion, have 

 been made by later observers. Bohm could find in Petromyzon ('88) 

 and the trout ('91) no radiations near the egg-nucleus after the 

 formation of the polar-bodies, while a beautiful sperm-aster is devel- 

 oped near the sperm-nucleus and divides to form the amphiaster. 

 Platner ('86) had already made similar observations in the snail 

 Arion, and the same result was soon afterward reached by Brauer 

 ('92) in the case of Branchipus, and by Julin ('93) in Styleopsis. 

 Pick's careful study of fertilization of the axolotl ('93) proved in 

 a very convincing manner not only that the amphiaster is a product 

 of the sperm-aster, but also that the latter is developed about the 

 middle-piece as a centre. The same result was indicated by Foot's 

 observations on the earthworm ('94), and it was soon afterward 

 conclusively demonstrated in echinoderms through the independent 

 and nearly simultaneous researches of myself on the egg of Toxo- 

 p7ieustes, of Mathews on Arbacia, and of Boveri on Echinus. Nearly 

 at the same time a careful study was made by Mead ('95, '98, i) of 

 the annelid Chcetopterus, and of the starfish Asterias by Mathews, 



^ '87. 2, p. 155. 



