190 MEAD. [VoL. XIV. 
the sperm-centrosomes — enter into the formation of the first 
cleavage-spindle. 
Miss Foot’s (97) account of the origin of the cleavage-cen- 
trosomes in Al/olobophora fetida does not accord with Boveri's 
theory of fertilization nor with that of Fol. ‘ The egg attrac- 
tion sphere is present during the two maturation divisions, but 
after the second polar body is formed and the female pronucleus 
begins to develop it totally disappears. The sperm attraction 
sphere is present until the head of the spermatozo6n begins to 
develop into the male pronucleus, when it also totally disappears. 
Both spheres are absent during a relatively long period (2.c., 
while the growing pronuclei are developing) ; and when the two 
pronuclei have attained their maximum size and are in contact, 
two attraction spheres again appear in the cytoplasm and the 
cleavage-spindle is formed.” 
Lillie (97) has observed a peculiar behavior of the centrosomes 
and asters in Unio. After undergoing extraordinary meta- 
morphoses, the egg-centrosomes alone enter into the formation 
of the cleavage-amphiaster, as they do in Myzostoma; a con- 
spicuous comet-like aster with a centrosome develops in con- 
nection with the sperm-nucleus, but totally disappears before 
the pronuclei come together. 
MacFarland’s (97) results on Pleurophyllidia are similar to 
those of Miss Foot on Allolobophora in that both egg- and sperm- 
centres are apparently absent during a certain period preceding 
the union of the pronuclei. 
All these observations form a serious obstacle in the way of 
accepting Boveri’s theory of fertilization, but there is a further 
and perhaps more serious difficulty in the insecurity of the 
hypotheses underlying the theory itself; wzz., (a) that the cen- 
trosome is a permanent organ of the cell, and (4) that it initiates 
and directs the cell-activities. We have already referred to the 
question of the validity of the first of these hypotheses (p. 185), 
and comparison of the normal fertilization phenomena of vari- 
ous forms leaves room for doubt as to the validity of the second. 
The eggs of various animals attain to different stages of matu- 
ration before fertilization takes place; some remain in the 
germinal-vesicle stage (with no visible centrosome or aster) 
