THE CENTROSOME IN FERTILIZATION 



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

 out beyond. 



We may now examine the question whether the sperm-centrosomes 

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

 the case is positively maintained in the case of Ascaris by Boveri, 

 Kostanecki, and Erlanger, in Physa by Kostanecki and Wierzejski 

 ('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 disappear, 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 Hertwig {Phyllirrhoe, Pterotrachea, '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 Prosthecer&us (Klinckowstrom, '97), in the mouse (Sobotta,. 

 '95), in Pleurophyllidia (MacFarland, '97), Physa (Kostanecki and: 

 Wierzejski, '96), Arenicola (Child, '97), Unto (Lillie, '97), Myzostoma. 

 (Kostanecki, '98), and Cerebratulus (Coe, '98). 1 Several of these 

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

 not only the asters but also tJie centrosomes 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 

 light 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 series 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 Myzostoma (according to Kostanecki) and 

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

 formation of the cleavage-spindle, and which, consequently, suffer no diminution in size. 

 Following these are the eggs of Chatopterus (Mead) and Ophryotrocha (Korschelt) 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 come the eggs of 

 Toxopneustes (Wilson) and Thalassema (Griffin), w r here the sperm-asters appear early and 

 develop to a very considerable size, but nevertheless become very much smaller and 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 exceedingly delicate 

 fibres remain. Those of Cerebratulus follow next. 



" Here the sperm-asters increase in size until they extend throughout 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 Pleuro- 

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

 tion, so that for a considerable period the egg is without trace of aster-fibres. Yet 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). 



