204 



FERTILIZATION OF THE OVUM 



entrance-path of the sperm-nucleus changes to the copulation-path. 

 It then begins to move slowly in a somewhat curved path toward the 

 meeting-point, often showing slight amoeboid changes of form as it 

 forces its way through the cytoplasm. From the meeting-point the 

 apposed nuclei move slowly toward the point of final fusion, which 

 in this case is near, but never precisely at, the centre of the Q.^g. 



These facts indicate that the paths of the germ-nuclei are deter- 

 mined by at least two different factors, one of which is an attraction 

 or other dynamical relation between the nuclei and the cytoplasm, 

 the other an attraction between the nuclei. The former determines 

 the entrance-path of the sperm-nucleus, while both factors probably 

 operate in the determination of the copulation-path along which it 

 travels to meet the egg-nucleus. The real nature of neither factor 

 is known. 



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 in plants). I have often observed in cases of dispermy in the sea-urchin, 

 that both sperm-nuclei move at an equal pace toward 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 char- 

 acter. Conklin (99) has recently suggested that the nuclei are drawn together by 

 the agency of protoplasmic currents in the egg-substance. 



3. Union of the Gcrm-miclei. 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 segincntation-nuclens. As early as 1881, however, 

 Mark clearly showed that in the slug Umax 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- 



