Eugene Howard Harper 377 



pursued the investigations still further, and by the discovery of fertili- 

 zation stages, was enabled to announce the origin of the much-discussed 

 yolk nuclei from spermatozoa. He thus not only accounted for the 

 origin of the merocytes, but established the fact of physiological poly- 

 spermy for the selachian group. He traced a continuous series of stages 

 from the entering sperm head to the fully "developed merocyte. He 

 obtained finally the conclusive evidence of their origin from spermatozoa 

 by determining that the dividing nuclei contained only one-half the 

 somatic number of chromosomes. Eiickert's results for selachians were 

 confirmed by Samassa, 95, Beard, 96, Sobotta, 96. His own complete 

 account appeared in 1899. 



The present state of the controversy which involves the ultimate fate 

 of the merocytes is outside of the province of this paper. The question 

 whether there may be another generation of yolk nuclei arising in late 

 cleavage stages, homologous with the periblast of teleosts, has been the 

 subject of controversy chiefly between His and Eiickert. 



The announcement of Eiickert of the origin of merocytes from sperma- 

 tozoa necessitated the modification of the prevalent assumption as to the 

 universally pathological nature of polj^spermy and opened up a field of 

 inquiry as to the causes of normal polyspermy; its adaptiveness ; its dif- 

 ference from the so-called pathological type; the influences which pre- 

 vent multiple conjugation with the egg nucleus ; the cause of the migra- 

 tion of the -supernumerary sperms into the yolk; their change from 

 mitotic to amitotic division, etc. The identification of these sperm nuclei 

 with the long Icnown " yolk nuclei," to which had been assigned by 

 common assumption the function of yollc digestion for the embryo, both 

 in selachians and teleosts, raised the question whether in reality in the 

 selachians the sperm nuclei have a normal or physiological role in 

 embryonic development. 



As mentioned above, the presence of nuclei in the yolk during the early 

 cleavage stages, forming a syncytium supposedly derived from the cleav- 

 age nuclei, had been used as an argument to support the theory of the 

 homology of the yolk of the selachian egg with the lower pole cells of 

 the frog-'s egg. 



Eiickert holds that the cause of polysj^ermy in the selachian egg is 

 simply the absence of protection against it, due to the thinness of the 

 egg membrane. The phenomena of conjugation of sperm nuclei with 

 each other and their multiple conjugation with the egg nucleus, seen 

 in the case of nicotinized eggs (Hertwig, 87), he holds to be due not to 

 polyspermy, 2)er se, but to changes brought about by nicotinization. Such 

 phenomena are absent in polyspermatic eggs, and he proposes a theory that 



