FERTILIZATION 187 



approach and meet in this region. As the nuclei come into 

 contact the two asters diverge in such a way as to lie at opposite 

 ends of a tangent drawn through the point of nuclear contact 

 (Figs. 94, 95). The nuclear walls then dissolve; a spireme 



o 



forms in each nucleus and segments, in each, into chromo- 

 somes, and these, as a typical spindle forms from the egg 

 cytoplasm, become arranged at its equator. The result is the 

 formation of a typical mitotic figure with s chromosomes. This 

 is the first cleavage figure, and here, for the first time in the 

 existence of the new organism, substances of paternal and 

 maternal origin are associated, on equal terms, in a common 

 structure. 



Before we mention any of the further details in the history 

 of this cleavage figure, we must return to consider briefly the 

 course of events in the fertilization of those eggs which are 

 already fully mature when the sperm cell enters (Echinoderms, 

 Ascidians). In such cases (Figs. 88, 93) the chief divergences 

 from the account just given result from the absence of any pause 

 of the sperm nucleus and middle piece after their entrance. 

 The egg nucleus is ready for fusion, and immediately upon the 

 entrance of the sperm the two nuclei proceed toward one 

 another as described above. The sperm nucleus thus does not 

 have time to be dissolved to any considerable extent, so that 

 when the two nuclei meet they are by no means of equal size 

 (Fig. 88), for the egg nucleus nearly always returns to its "rest- 

 ing" state after its maturation is completed. Nevertheless it is 

 known from their history that the two nuclei are equivalent in 

 chromosomal composition. Frequently, too, the centrosome 

 does not divide until just as the two nuclei meet, or even after 

 they have begun to fuse. The two centrosomes accompanied 

 by asters then move to opposite poles of the combined nuclei 

 and there establish the mitotic figure. The sperm nucleus in 

 these cases does not become resolved into a typical nuclear 

 condition until after its fusion with the egg nucleus. It often 

 results from this that the two nuclear substances seem to mingle 

 quite completely before the spindle is formed and it is not so 



