UNION OF THE GERM-CELLS 149 



i. Immediate Results of Union 



The union of the germ-cells calls forth profound changes in both. 



(a] The Spermatozoon. Almost immediately after contact the tail 

 ceases its movements. In some cases the tail is left outside, being 

 carried away on the outer side of the vitelline membrane, and only 

 the head and middle-piece enter the egg (echinoderms, Fig. 74). 

 In other cases the entire spermatozoon enters (amphibia, earthworm, 

 insects, etc., Fig. 64), but the tail always degenerates within the 

 ovum and takes no part in fertilization. Within the ovum the 

 sperm-nucleus rapidly grows, and both its structure and staining- 

 capacity rapidly change (cf. p. 127). The most important and signifi- 

 cant result, however, is an immediate resumption by the sperm-micleus 

 and spcrm-centrosome of tJie povver of division which has hitherto 

 been suspended. This is not due to the union of the germ-nuclei ; 

 for, as the Hertwigs and others have shown, the supernumerary 

 sperm-nuclei in polyspermic eggs may divide freely without copu- 

 lation with the egg-nucleus, and they divide as freely after entering 

 enucleated egg-fragments. The stimulus to division must therefore 

 be given by the egg-cytoplasm. It is a very interesting fact that in 

 some cases the cytoplasm has this effect on the sperm-nucleus 

 only after formation of tJie polar bodies ; for when in sea-urchins the 

 spermatozoa enter immature eggs, as they freely do, they penetrate 

 but a short distance, and no further change occurs. 



(b) T/ie Ovum. The entrance of the spermatozoon produces an 

 extraordinary effect on the egg, which extends to every part of its 

 organization. The rapid formation of the vitelline membrane, already 

 described, proves that the stimulus extends almost instantly through- 

 out the whole ovum. 1 At the same time the physical consistency of 

 the cytoplasm may greatly alter, as for instance in echinoderm eggs, 

 where, as Morgan has observed, the cytoplasm assumes immediately 

 after fertilization a peculiar viscid character which it afterwards 

 loses. In many cases the egg contracts, performs amoeboid move- 

 ments, or shows wave-like changes of form. Again, the egg-cyto- 

 plasm may show active streaming movements, as in the formation of 

 the entrance-cone in echinoderms, or in the flow of peripheral proto- 

 plasm towards the region of entrance to form the germinal disc, as in 

 many pelagic fish-eggs. An interesting phenomenon is the formation, 

 behind the advancing sperm-nucleus, of a peculiar funnel-shaped mass 

 of deeply staining material extending outwards to the periphery. 

 This has been carefully described by Foot ('94) in the earthworm, 



1 I have often observed that the formation of the membrane, in Toxopiieustes, proceeds 

 like a wave from the entrance-point around the periphery, but this is often irregular. 



