UNION OF THE GERM-CELLS 153 



Hertwig first called attention to the fact which is easy to observe in the living 

 sea-urchin egg that the egg-nucleus does not begin to move until the sperm- 

 nucleus has penetrated some distance into the egg and the sperm-aster has attained 

 a considerable size ; and Conklin ('94) has suggested that the nuclei are passively 

 drawn together by the formation, attachment, and contraction of the astral rays. 

 While this view has some facts in its favour, it is, I believe, untenable, for many 

 reasons, among which may be mentioned the fact that neither the actual paths 

 of the pro-nuclei nor the arrangement of the rays support the hypothesis ; nor does 

 it account for the conjugation of nuclei when no astral rays are developed (as in 

 Protozoa), or are insignificant as compared with the nuclei (as in plants). I have 

 often observed in cases of dispermy in the sea-urchin, that both sperm-nuclei move 

 at an equal pace towards the egg-nucleus ; but if one of them meets the egg-nucleus 

 first, the movement of the other is immediately retarded, and only conjugates with 

 the egg-nucleus, if at all, after a considerable interval ; and in polyspermy, the egg- 

 nucleus rarely conjugates with more than two sperm-nuclei. Probably, therefore, 

 the nuclei are drawn together by an actual attraction which is neutralized by union, 

 and their movements are not improbably of a chemotactic character. 



3. Union of the Germ-nuclei. The Chromosomes 



The earlier observers of fertilization, such as Auerbach, Stras- 

 burger, and Hertwig, described the germ-nuclei as undergoing a com- 

 plete fusion to form the first embryonic nucleus, termed by Hertwig 

 the cleavage- or segmentation-nucleus. As early as 1881, however, 

 Mark clearly showed that in the slug Limax this is not the case, the 

 two nuclei merely becoming apposed without actual fusion. Two 

 years later appeared Van Beneden's epoch-making work on Ascaris, 

 in which it was shown not only that the nuclei do not fuse, but that 

 they give rise to two independent groups of chromosomes which 

 separately enter the equatorial plate and whose descendants pass 

 separately into the daughter-nuclei. Later observations have given 

 the strongest reason to believe that, as far as the chromatin is con- 

 cerned, a true fusion of the nuclei never takes place during fertiliza- 

 tion, and that the paternal and maternal chromatin may remain 

 separate and distinct in the later stages of development possibly 

 throughout life (p. 219). In this regard two general classes may be 

 distinguished. In one, exemplified by some echinoderms, by AmpJii- 

 v.rus, Phallusia, and some other animals, the two nuclei meet each 

 other when in the reticular form, and apparently fuse in such a manner 

 that the chromatin of the resulting nucleus shows no visible distinc- 

 tion between the paternal and maternal moieties. In the other class, 

 which includes most accurately known cases, and is typically repre- 

 sented by Ascaris (Fig. 65) and other nematodes, by Cyclops (Fig. 72), 

 and by PterotracJiea (Fig. 68), the two nuclei do not fuse, but only 

 place themselves side by side, and in this position give rise each to 

 its own group of chromosomes. On general grounds we may confi- 



