2O2 FERTILIZATION OF THE OVUM 



tozoon the germinal vesicle moves toward the periphery, its membrane 

 fades away, and a radially directed mitotic figure appears, by means 

 of which the first polar body is formed (Fig. 97). Meanwhile the 

 protoplasm flows toward the upper pole, the peri-vitelline zone disap- 

 pears, and the egg now shows a sharply marked polar differentiation. 

 A remarkable phenomenon, described by Whitman in the leech ('78), 

 and later by Foot in the earthworm ('94), is the formation of " polar 

 rings," a process which follows the entrance of the spermatozoon 

 and accompanies the formation of the polar bodies. These are two 

 ring-shaped cytoplasmic masses which form at the periphery of the 

 egg near either pole and advance thence toward the poles, the upper 

 one surrounding the point at which the polar bodies are formed 

 (Fig. 102). Their meaning is unknown, but Foot ('96) has made 

 the interesting discovery that they are probably of the same nature 

 as the yolk-nuclei (p. 1 56). 



2. Paths of the Germ-nuclei (Pro-nuclei) 1 



After the entrance of the spermatozoon, both germ-nuclei move 

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

 traversed by them 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 egg and then 

 pauses while the germinal vesicle moves toward the periphery, and 

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

 cant 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 

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

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

 into the egg forming a considerable angle, with its " copulation-path " 

 toward the egg-nucleus. 



These facts are. well illustrated in the sea-urchin egg (Fig. 103), 

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

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



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

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

 Hertwig's terms egg-nucleus and sperm-nucleus, 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. 275), 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. 243). 



