KARYOKINESIS. 29 



which the cleavage centrosomes are undoubtedly derived from the sperm centrosome. 

 As to Physa, in which this origin is strenuously maintained, Kostanecki's figures are 

 capable of another interpretation than that which he puts upon them. All of his 

 figures which show the two germ nuclei and the two centrosomes up to the time 

 when the latter have taken their final position at the poles of the nuclei (his fig. 

 %oa) show one centrosome in connection with each nucleus and nowhere in these 

 stages is a central spindle shown, except in fig. 30, which shows a single fibre con- 

 tinuous from pole to pole ; even in the later stage, fig. 33«, there is no central 

 spindle. Further, it is a significant fact that when the egg centrosome disappears 

 the sperm centrosome also disappears (fig. 25-28), while the next stage figured (fig. 

 30) shows two lai'ge and well marked centrosomes, and in fig. 31 one of these lies 

 in close connection with each of the germ nuclei. 



In Pleurophylidia, according to MacFarland, the sperm asters and centrosomes 

 disappear completely during the formation of the second jiolar body and for a rela- 

 tively long period no centrosomes are present. After the germ nuclei are in con- 

 tact the cleavage centrosomes appear, and since they frequently occup}' positions 

 similar to the sperm centers, the author thinks they are derived from these. 



In Ujiio Lillie finds that both egg and sjoerm centrosomes and asters completely 

 disappear and that accessory centrosomes and asters also arise and disappear. 

 Finally the two cleavage centrosomes arise independantly of each other and of any 

 of their predecessors. 



In Limncea Linville finds that both egg and sperm centrosomes disappear for 

 a time, but since the cleavage spindle first involves the sperm nucleus, he concludes 

 that the cleavage centrosomes are of spermatic origin. His figures, howevei", do not 

 bear out this interpretation ; fig. 6 shows the incipient cleavage spindle in connection 

 with what is surely the egg nucleus, though he calls it the fused germ nuclei, (so 

 far as I am aware the germ nuclei do not fuse in any mollusk.) Fig. 18, which is 

 one of the earliest of his figures showing the cleavage centrosomes, shows one in con- 

 nection with each germ nucleus and with no central spindle between them. 



Boveri's figure of Pterotrachea ('90, fig. 10), which is so widely copied in the 

 text-books, shows one centrosome in connection with each germ nucleus and no 

 central spindle between the two. 



In other groups of animals the evidence in favor of Boveri s hypothesis is by 

 no means conclusive, while much positive evidence has been brought against it. 

 Among Turbellaria I know of no single case clearly favorable to this view ; (cf. 

 Klinckowstnim '9G), Van der Stricht ('98), Gardiner ('99), Van Name ('99). Coe's 

 ('99) work on Cerebratiilus affords very good evidence that the sperm centers 

 become the cleavage centers in that animal, and the same is true of Chcetoplerus 

 (Mead '97), and of Thalasse7na (M'illin ('90). On the other hand Foot ('97) has 

 shown in a convincing manner that the cleavage centrosomes are new formations in 

 Allolobopliora and Child ('98) holds the same position with regard to Aroiicola. 



If all these accounts are to be believed, therel'ore, the cleavage centrosomes may 

 come from the sperm, from the egg, or from both, and it is at once apparant that 



