THE FERTILISATION OF THE EGG 



23 



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Fig. 7. Egg- of a sea urchin, treat- 

 ed with ether. The pronuclei have 

 not fused. The egg nucleus {^) 

 forms a monaster, the sperm 

 nucleus (d") an amphiaster. After 

 Wilson, 



This was observed, e.g., by 

 Ziegler (1898) when, in his 

 experiments with fragmented 

 sea urchin eggs, the nuclei of 

 egg and sperm were isolated 

 in different fragments. Wil- 

 son (1901), by preventing 

 the fusion of the pronuclei in 

 sea urchin eggs by means of 

 an ether treatment, observed 

 the same phenomenon (Fig. 

 7). In Amphibia, the same 

 thing may occur in unripe or 

 overripe eggs, in which the 

 fusion of the pronuclei is 

 retarded by the abnormal 



condition of the protoplasm (Bataillon and Tchou Su, 1934). 

 (3) The co-operation of the chromatin of the sperm nucleus 

 is not indispensable for a normal bipolar mitosis. This is proved 

 by some cases of heterogeneous fertilisation, in which the sperm 

 penetrates into the egg, and male and female pronuclei unite, 

 but the chromatin of the sperm nucleus remains entirely 

 compact. In the course of a mitosis which in all other respects 

 is normal, this chromatin does not divide into chromosomes, and 

 is soon afterwards extruded into the cytoplasm, and resorbed. 

 This occurs, e.g., in the case of fertilisation of sea urchin eggs 

 with sperm of the mussel Mytilus (Kupelwieser, 1908) (Fig. 8). 

 The same may happen after fertilisation with sperm damaged by 

 radium irradiation (O., G., and P. Hertwig) or trypaflavin treat- 

 ment (Dalcq). (4) Cytasters, produced e.g. in sea urchin eggs, 

 by treatment with hypertonic sea water, are usually unable to 

 divide, and therefore remain monocentric. If, however, they are 

 situated in the neighbourhood of a normal mitosis, they can 

 sometimes "capture" part of the nuclear material of the latter. 

 Thereupon they divide, and form an amphiaster (Fry, 1925). 

 Working with sperm treated with trypaflavin, Dalcq obtained 

 amphiasters which, as a consequence of irregularities in the 

 course of division, contained no chromosomes, but only part 



