CENTROSOME AND ARCHOPLASM IN FERTILIZAJ^ION 159 



ization of the gasteropod Physa, and have reached exactly the same 

 result as that obtained in the echinoderms. Here also the egg-centre 

 degenerates, and its place is taken by a centrosome brought in by 

 the spermatozoon and giving rise to a sperm-amphiaster, which per- 

 sists as the cleavage-amphiaster (Fig. 64). A strong presumption is 

 thus 'created that Conklin was in error; and if this be the case, the 

 last positive evidence of a conjugation of centrosomes in the animal 

 egg disappears. 1 



In view of this result we may well hesitate to accept Guignard's 

 conclusions in the case of flowering plants. The figures of this 

 author show in the clearest manner four centrosomes lying in the 

 neighbourhood of the apposed, germ-nuclei (Fig. 80) ; but the conju- 

 gation of these centrosomes was an inference, not an observed fact, 

 and has not been confirmed by any subsequent observer. Until such 

 confirmation is forthcoming we must receive Guignard's results with 

 scepticism. 2 



The third view, based upon the single case of Myzostoma as 

 described by Wheeler ('95), apparently rests on strong evidence, 

 though its force cannot be exactly estimated until a more detailed 

 account has been published. In this case no sperm-aster can be 

 seen at any period, with which is correlated the fact that no middle- 

 piece can be made out in the spermatozoon. The egg-centrosome, 

 on the other hand, is stated -to persist after the formation of the 

 second polar body, to become double at a very early period, and 

 to give rise directly to the cleavage-amphiaster (Fig. 78). I can find 

 no ground in Professor Wheeler's paper to doubt the accuracy of 

 his conclusions. Nevertheless, an isolated case, which stands in 

 contradiction to all that is known 'of other forms, must rest on irre- 

 fragable evidence in order to command acceptance. Since, more- 

 over, the case involves the whole theory of fertilization based on 

 other animals (cf. p. 141), it must, I think, await further investiga- 

 tion. 



1 Richard Hertwig has, however, recently published a very interesting observation which 

 indicates that we may not yet have fully fathomed the facts in the case of echinoderms. If 

 unfertilized echinoderm-eggs, after formation of the polar-bodies, lie for many hours in 

 \vater or lie treated with dilute poisons (strychnine), they may form a more or less perfectly 

 developed amphiaster, and the nucleus may even make an abortive attempt at division. No 

 centrosomes, however, could be discovered, even by the most approved methods. This 

 remarkable phenomenon is probably of the same nature as the formation of artificial asters 

 observed by Morgan (p. 226), but its meaning is not clear. 



2 Van der Stricht, in a recent paper on Amphioxus ('95), is inclined to believe that a 

 fusion between the egg-centre and the sperm-centre occurs; but the evidence is very incom- 

 plete, and a comparison with the case of Physa indicates that his conclusion cannot be 

 sustained. The same criticism applies to the earlier work of Blanc ('91, '93) on the trout's 

 egg- 



