UNION OF THE GERM-CELLS 151 



2. Paths of the Gcrui-nnclei {Pro-nuclei) ^ 



After the entrance of the spermatozoon both germ-nuclei move 

 through the egg-cytoplasm and finally meet one another. The paths 

 traversed by each vary widely in different forms. In general two 

 classes are to be distinguished, according as the polar-bodies are 

 formed before or after entrance of the spermatozoon. In the former 

 case (echinoderms) the germ-nuclei unite at once. In the latter case 

 the sperm-nucleus advances a certain distance into the tgg and then 

 pauses while the germinal vesicle moves towards the periphery, and 

 gives rise to the polar-bodies (Ascaris, annelids, etc.). This significant 

 fact proves that the attractive force between the two nuclei is only 

 exerted after the formation of the polar-bodies, and hence that the 

 entrance-path of the sperm-nucleus is not determined by such at- 

 traction. A second important point, first pointed out by Roux, is 

 that the path of the sperm-nucleus is curved, its " entrance-path " 

 into the Q.gg forming a considerable angle with its "copulation-path " 

 towards the egg-nucleus. 



These facts are well illustrated in the sea-urchin ^gg (Fig- 77), 

 where the egg-nucleus occupies an eccentric position near the point 

 at which the polar bodies are formed (before fertilization). Entering 

 the Qgg at any point, the sperm-nucleus first moves rapidly inward 

 along an entrance-path that shows no constant relation to the position 

 of the egg-nucleus and is approximately but never exactly radial, i.e. 

 towards a point near the centre of the ^g%. After penetrating a 

 certain distance its direction changes slightly to that of the copulation- 

 path, which, again, is directed not precisely towards the egg-nucleus, 

 but towards a meeting-point where it comes in contact with the 

 egg-nucleus. The latter does not begin to move until the entrance- 

 path of the sperm-nucleus changes to the copulation-path. It then 

 begins to move slowly in a somewhat curved path towards 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 ^gg. 



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



1 The terms "female pro-nucleus," "male pro-nucleus" (Van Beneden), are often ap- 

 plied to the germ-nuclei before their union. These should, I think, be rejected in favour of 

 Hertwig's terms egg-niuleiis -icnA sperin-mideus, on two grounds: (i) The germ-nuclei are 

 true nuclei in every sense, differing from the somatic-nuclei only in the reduced number of 

 chromosomes. As the latter character has recently been shown to be true also of the 

 somatic nuclei in the sexual generation of plants (p. 196), it cannot be made the ground for 

 a special designation of the germ-nuclei. (2) The germ-nuclei are not male and female 

 in any proper sense (p. 183). 



