252 LILLIE. [Vol. XVII. 



or lack of relationship to anything definite, before their dis- 

 appearance. The case of Arenicola (Child, '98) gives especially 

 clear evidence of independent origin of the cleavage centers, 

 for in this case the sperm-centrosomes do not disappear until 

 after the formation of the second polar body, and the cleavage 

 centrosomes arise soon after in such a position that it seems 

 simply impossible that they should represent persistent sperm- 

 centrosomes. Child himself is " inclined to regard the cleav- 

 age centrosomes as new formations and as not related to the 

 ' male ' centrosome." In the other cases there is certainly 

 no objective evidence of identity of sperm-centrosomes and 

 cleavage centrosomes, indeed much that speaks against it. 



I agree, therefore, with Wheeler and with the views ex- 

 pressed by Korschelt (95b, p. 655) and Brauer ('93) that the 

 centrosomes are of no special significance in fertilization. The 

 cleavage centers may arise from the sperm-centers, or from 

 the egg-center (doubtfully, ]\Iyzosto7na} Wheeler, '97), or in 



1 I cannot regard Kostanecki's observations on Myzostoma {'98) as proving 

 the origin of the cleavage centers from the sperm-centrosomes in this case. 

 According to both his observations and Wheeler's ('97) no sperm-aster is visi- 

 ble (until at least after the formation of the second polar body, Kostanecki), 

 although the spermatozoon enters the egg before the breaking down of the ger- 

 minal vesicle. The sperm-nucleus always lies toward the vegetative pole. The 

 germ-nuclei enlarge simultaneously, and begin to approach, the sperm-nucleus mak- 

 ing a more extensive migration than the egg-nucleus, so that their place of meeting 

 is always nearer the animal pole. When the nuclei have approached within a short 

 distance of each other, there appears between them a usually double aster (source 

 of cleavage centrosomes). Sobotta's observations on the mouse are entirely similar. 



W^hat is the origin of these centrosomes ? The author (Kostanecki) sees only 

 two possibilities : either they are the egg-centrosomes reappearing, or the sperm- 

 centrosomes. 



" Ich habe aber iiberdies vereiitzelte Bilder bei Myzostoma gesehen, die direct 

 fiir die Herkunft der Centrosomen von Spermatozoon sprechen : wenn die beiden 

 Geschlechtskeme die Gestalt von grosseren Blaschen angenommen haben, konnte 

 ich an der Eimitte zugekehrten Seite des Spermakerns bistueilen die Andeuimig 

 einer Strahlung sehen, die jedoch mehr durch eine radiare Anordung der kleinen 

 Dotterkornchen zum Ausdruck kam. Die radiar angeordneten Kornchen umga- 

 ben ein helleres Feld, in dessen Mitte ein oder zwei kleine Punkte zu sehen 

 waren. ... Ich glaube in alien diesen Fallen (PI. XXIV, Fig. 14; PI. XXV, Figs. 

 15, 16) das Spermacentrosoma oder die Spermacentrosomen vor Augen gehabt zu 

 haben, wenn auch in Anbetracht des Umstandes, dass das Hauptkriterium fiir 

 die Existenz von Centrosomen, namlich eine deutliche Strahlung, hier fehlte, even- 

 tuelle Zweifel nicht mit absoluter Sicherheit zuriickgewiesen werden konnen." 



But these " doubtful " sperm-centrosomes were never traced to the cleavage 



