THE CENTROSOME I^f FERTIIIZATJOX 21 3 



though in substance it would still retain an element of truth, as pointed 

 out beyond. 



We may now examine the question whether the sperm-centrosomcs 

 are actually identical with the cleavage-centrosomes. That such is 

 the case is positively maintained in the case of Ascaris by I^overi 

 Kostanecki, and Erlanger, in PJiysa by Kostanecki and \Vierzejski 

 ('96), in Thalassema by Griffin ('96, '99), and in Chcetoptcrus by Mead 

 ('95, '98). The two last-mentioned observers, who have followed 

 the phenomena with especial care, produce very strong evidence 

 that at no time do the sperm-centrosomes and asters disa])pear, and 

 that the former may be traced in unbroken continuity from the time 

 of their first appearance to the daughter-cells resulting from the first 

 cleavage (Figs. 99, 155). On the other hand, a considerable number 

 of observers, beginning with W^x\.\V\g{PhyllirrJioc\ Ptcrotmchca, '75), 

 have found that as the sperm-nucleus enlarges the sperm-asters di- 

 minish in size, until, in many cases, they nearly or quite disappear ; for 

 example, in /^r^i-///^f^r^;/j-(KHnckowstrom, '97), in the mouse (Sobotta, 

 '95), in PlciiropJiyllidia (MacFarland, '97), Pliysa (Kostanecki and 

 Wierzejski, '96), Arenicola {Q\i\\(\i, '97), 6';//^ (Lillie, '97), Myzostoma 

 (Kostanecki, '98), and Cerebmtulus (Coe, '98).^ Several of these 

 observers (KUnckowstrom, MacFarland, Lillie, Child) have found that 

 not only the asters but also the ccntrosomes totally disappear about 

 the time the germ-nuclei come together, a new pair of cleavage- 

 centrosomes and asters being afterward developed at the poles of 

 the united nuclei. These conclusions, if correct, place in a new 

 Hght the disappearance of the egg-centrosomes ; for this process 



1 Coe has pointed out that the eggs of various animals may be arranged in a scries show- 

 ing successive graduations in the disappearance of the sperm-asters. " At the head of the 

 series we must place the eggs of Ascaris and I\[yzostouia (according to Kostanecki) and 

 similar ones in which the sperm-asters make their appearance only a short time befitrc the 

 formation of the cleavage-spindle, and which, consequently, suffer no diminutit)n in si/c. 

 Following these are the eggs of Chixtopterus (Mead) and Ophryotrocha (Korschclt) and 

 of some echinoderms in which the sperm-asters develop very early, but are not described 

 as decreasing in size before the formation of the cleavage-spindle. Then cume the eggs of 

 Toxopnensies (Wilson) and Thalassema (Griffm), where the sperm-asters appear early and 

 develop to a very considerable size, but nevertheless become very much smaller ant! less 

 conspicuous after the germ-nuclei have come together. After these we must place the eggs 

 of Physa (Kostanecki and Wierzejski), for here the sperm-asters, after becoming very large 

 and conspicuous, degenerate to such an extent that only a very few exceeilingly delicate 

 fibres remain. Those of Cerebratulus follow next. 



" Here the sperm-asters increase in size until they extend thrt>ughout the whole body 

 of the cell, but at the time of fusion of the germ-nuclei they degenerate completely. The 

 peripheral portions of their fibres, however, may be followed, as stated above of PUuro- 

 phyllidia, Prosthecerccus, etc., where the sperm-asters degenerate soon after their forma- 

 tion, so that for a considerable period the tg^ is without trace of aster-llbres. Vet in all 

 of those cases where the sperm-asters disappear and their centrosomes become lost among 

 the other granules of the cell, we are justified in believing that the sperm-centrosomes 

 nevertheless retain their identity, and later reappear in the cleavage-asters " ('98, p. 455). 



