194 FERTILIZATION OF THE OVUM 



several later observers have shown that egg-fragments, obtained by 

 shaking eggs to pieces, are readily penetrated by the spermatozoa, 

 and that such fragments, though containing no nuclear matter from 

 the egg. may segment and give rise to perfect larvae. 1 (2) Boveri 

 ('88) has observed that in ordinary fertilization the sperm-aster may 

 separate from the sperm-nucleus, travel through the cytoplasm to the 

 egg-nucleus and cause cleavage, the sperm-nucleus afterward fusing 

 with one of the nuclei of the two-cell stage (" partial fertilization "). 

 (3) Most remarkable of all, Boveri, confirmed by Ziegler ('98), has 

 recently observed that during the first cleavage the whole of the 

 chromatin may pass to one pole, so that upon division one of the 

 halves of the egg receives only a centrosome without a nucleus. In 

 the nucleated half cleavage proceeds as usual. In the enucleated 

 half the centrosomes and asters continue for a considerable period 

 to multiply at the same rate as the cleavage of the nucleated half, 

 though the cell-body does not itself divide. 2 Putting these facts 

 together we must conclude (i) that something is introduced into the 

 egg by the middle-piece of each spermatozoon entering it that is 

 either a centrosome or has the power to incite the formation of one ; 

 (2) that the centrosome thus arising is structurally independent of 

 both nuclei and may divide independently of them ; (3) that indepen- 

 dently of the division of the nucleus or cell-body there is some kind 

 of historical continuity between the centrosomes of successive genera- 

 tions. 



In the case of echinoderm-eggs this continuity is not yet known to 

 be effected by actual persistence of the centrosomes. 3 There are, 

 however, a number of cases in which the division of the primary 

 cleavage-centrosomes and the persistence of their descendants as 

 those of the daughter-cells seem to have been conclusively shown 

 for example on Ascaris (Van Beneden, Boveri, Kostanecki, and Sied- 

 lecki), in the trout (Henneguy, '96), in Thalassema (Griffin, '96, '99), 

 in Ch&topterus (^A.saA.1 '95, '98), in Physa (Kostanecki and Wierzejski, 

 '96), in Cerebratulus (Coe, '98), and in Rhynchelmis (Vejdovsky and 

 Mrazek, '98). In Thalassema and Cerebratulus (Figs. 99, 155) the 

 centrosome is a minute granule at the focus of the sperm-aster, 

 which divides to form an amphiaster soon after the entrance of the 

 spermatozoon. During the early anaphase of the first cleavage, each 

 centrosome divides into two, passes to the outer periphery of the 

 centrosphere, and there forms a minute amphiaster for the second 



1 Cf. p. 353- 2 Cf. p. 108. 



3 Erlanger's statement ('98) that the centrosomes persist through the first cleavage in 

 echinoderm-eggs is not supported by his figures ; and I am convinced from my own long- 

 continued studies of these eggs, as well as by an examination of Erlanger's preparations, 

 kindly placed in my hands by Professor Bxitschli, that these difficult objects are very unfavour- 

 able for a decision of the question. 



