226 EDWIN G. CONKLIN. 



or both of its poles with those of the egg spindle. Since under 

 normal conditions one of the cleavage centrosomes of Crepidtila 

 arises in connection with the egg and the other with the sperm 

 nucleus the only respect in which these experimental results 

 differ from the normal is that in the former the centrosomes 

 divide while the nuclei are still far apart thus giving rise to two 

 spindles or to a tetraster, whereas in the latter these centrosomes 

 do not appear until after the germ nuclei and spheres have met 

 and they do not divide until the prophase of the second cleavage. 



In such a case as this we have essentially the same phenomenon 

 as is found in dispermic echinoderm eggs (Driesch, Boveri, 

 Wilson) save that in the former the two additional centrosomes 

 are not derived from an additional spermatozoon but come from 

 the egg centrosome. Of this fact there cannot be a particle of 

 doubt and it seems to me to shed light upon the much discussed 

 question as to the source of the cleavage centrosomes of differ- 

 ent animals under normal conditions. 



It has evidently been a mistake to suppose that the cleavage 

 centrosomes could arise in but a single way and that all animals 

 must conform to this single type. Under experimental condi- 

 tions the cleavage centrosomes may arise in one and the same 

 animal in connection with the sperm nucleus, in connection with 

 the egg nucleus or, as I have shown in connection with both of 

 these nuclei, while it is possible that they may arise dc novo 

 anywhere in the egg cytoplasm, though this latter view is by no 

 means so well supported by evidence as are the former ones 

 (see Conklin 1902). It is highly probable therefore that the 

 source of the cleavage centrosomes may differ in different ani- 

 mals, or even in the same animal under different conditions. 



It is interesting to note that this whole discussion as to the 

 supposed importance of the source of the cleavage centrosomes 

 had its origin in the thought that the sex cells were in them- 

 selves incomplete and incapable of development save as each com- 

 plemented the other (Boveri '87, '91), or in the rival notion that 

 the centrosomes were the bearers of heritable qualities (Fol '91). 

 In the light of recent experimental work both of the these views 

 are seen to be untenable and the subject has therefore lost most 

 of its interest and significance. 



