IV. i INITIAL STRUCTURE OF THE GERM 251 



(Kostanecki and Wierzejski) : the 'polar rings' appear in 

 Allolobophora, Clepsine, and Rliynchelmis (K. Foot, Whitman, 

 Vejdovsky), while in Teleostei the periblastic hyaline layer 

 becomes concentrated as the blastodisc at the animal pole. The 

 case of Dentalinm has been already referred to (p. 223, Fig. 133). 

 Although the cases in which the r61e played by the spermato- 

 zoon in determining egg and embryonic structure are, as has been 

 observed, not very numerous, yet it is fully to be expected that 

 renewed investigation will show that some such rearrangement 

 of the materials of the egg is in many, perhaps in all, instances 

 one of the first results of fertilization. 



LITERATURE 



A. AGASSIZ. Embryology of the Ctenophorae, Mem. Amer. Acad. Arts 

 and Sciences, N.S. x, 1873. 



E. G. CONKLIN. The organization and cell-lineage of the Ascidian egg, 

 Journ. Ac. Nat. Sci. Philadelphia, xiii, 1905. 



K. FOOT. Preliminary note on the maturation and fertilization of the 

 egg of Alloldbophora foetida, Journ. Morph. ix, 1894. 



T. T. GROOM. On the early development of Cirripedia, Phil. Trans. 

 Boy. Soc. clxxxv, B. 1894. 



K. VON KOSTANECKI and A. WIEKZEJSKI. Ueber das Verhalten der 

 sogen. achromatischen Substanzen im befruchteten Ei (Physa fontinalis), 

 Arch. Hiikr. Anat. xlvii, 1896. 



A. LANG. Fauna und Flora des Golfes von Neapel : XI. Die Poly- 

 claden, Leipzig, 1884. 



W. Roux. Die Bestimmung der Medianebene des Froschembryo 

 durch die Copulationsrichtung des Eikernes und des Spermakernes, Arch, 

 mikr. Anat. xxix, 1887 ; also, Ges. Abh. 21, Leipzig, 1885. 



F. VEJDOVSKY. Entwickelungsgeschichtliche Untersuchungen, Prag, 

 1888-92. 



C. 0. WHITMAN. The embryology of Clepsine, Quart. Jotim. Micr. Sci. 

 xviii, 1878. 



E. B. WILSON and A. P. MATHEWS. Maturation, fertilization and 

 polarity in the Echinoderm egg, Journ. Morph. x, 1895. 



13. WHAT PART DOES THE NUCLEUS PLAY IN 



DIFFERENTIATION ? x 



We have already criticized and rejected the hypothesis that 

 the division of the nucleus during segmentation is a qualitative 

 process. It may still be urged, however, that the nucleus is not 

 insignificant in differentiation. In support of this contention 

 the following arguments have been put forward. 



1. It is urged that the nucleus is essential for the life of the cell, 

 1 See also Appendix B. 



