212 FEKTILIZATIOX 01- THE OVUM 



the egg identical with, or the descendants of, a centrosome or pair 

 of centrosomes in the middle-piece of the spermatozoon ? Second, 

 do they actually jiersist to form those of the cleavage-amphiaster ? 

 In the present state of kno\vledi;e we are not in a position to give an 

 aflfirmative answer to the lirst of these C|uestions. As has been shown 

 in Chapter III., it is no longer possible to doubt that the middle- 

 piece either contains or is itself a metamorphosed centrosome ; but, as 

 pointed out at page 196, it does not seem possible that the extremely 

 minute centrosome of the sperm-aster can represent the entire cen- 

 trosome of the middle-piece (however we conceive the origin of the 

 latter). At most we can only assume that a part of the latter per- 

 sists as the sperm-centrosome within the egg. The exact origin of 

 the latter still remains problematical. A large number of observers 

 are now agreed that the sperm-aster is formed about a focus that is 

 either in or very near the middle-piece ; ^ but no one, I believe, has 

 yet succeeded in showing that the centrosome actually is the meta- 

 morphosed middle-piece, or escapes from it.^ The possibility there- 

 fore remains that the centrosome of the sperm-aster is, not actually 

 imported as such into the ^2,g, but is either only a portion of the 

 original spermatid-centrosome, or, as was first suggested by Miss Foot 

 ('97) and further discussed by Mead {'98, 2), is, like the aster, formed 

 anew in the egg-cytoplasm. If the latter alternative be the case, the 

 original form of Boveri's hypothesis would have to be abandoned ; 



1 For example, in echinoderms (Flemming, '81, O. and R. Hertvvig, '86, Boveri, '95, 

 Wilson and Mathews, '95, Hill, '95, Reinke, "95, R. Hertwig, '96. Dofiein, '97, 2, Eilanger, '98), 

 in Pterotrachea and Pieris (Henking, '91, '92), in the axolotl (Fick, '93), and Triton 

 (Michi-elis, • '97), in Phalliisia (Hill, '95), in Ophryotrocha (Korschelt, '95), in Physa 

 (Kostanecki and Wierzejski, '96), in Strojigylus (Meyer, '95), in Thysanozoon (Van der 

 Stricht, '98), and Prosthiostoinum (Francolte, '98). In a large numljer of other cases the 

 sperm-aster is found near the sperm-nucleus, but its relation to the middle-piece has not 

 been demonstrateil. 



- I myself formerly concluded ('95, 2) that the entire middle-piece of echinoderms is the 

 centrosome — a result apparently confirmed in a most positive manner by Erlanger ('98), 

 as well as by R. Hertwig ('96) and Dofiein ('97, 2). I have, however, demonstrnted this 

 to be an error, showing that the extremely minute centrosome is quite distinct from the 

 middle-piece, the latter being thrown aside and degenerating in the egg-cytoplasm outside 

 of the newly formed sperm-aster (Figs. 12, 94). This fact, of which the phenomena in 

 ToxopneusUs leave no doubt (see Wilson, '97, '99), is, I think, fatal to Kostanecki's and 

 Wierzejski's theory of fertilization ('96, pp. 374-375), according to which the archoplasm of 

 the middle-piece gives rise to the new astral system and is thus the essential fertilizing sub- 

 stance (the centrosome being merely a mechanical centre for the attachment of the rays) ; 

 but the most careful examination has still failed to show whether the centrosome actually 

 escapes from the middle-piece, nor have other observers had better success with any animal. 

 Erlanger ('96, 2, '97, 4) believes he has seen the centrosome in the Ascaris spermatozoon 

 as a distinct body lying behind the nucleus, and that it can be traced continuously into the 

 egg and after its division into the two poles of the cleavage-figure. Neither the schematic 

 figures of his preliminary nor the photographic ones of his final paper seem sufficient to 

 establish either the identity or the subsequent history of the granule in cjuestion. 



