64 Indications of a Central Zone of Development 



foetus of a half-calf, in which all the posterior part of 

 the trunk was missing as if it had been removed by a 

 transverse cut. 36 



However that may be, one fact which speaks in favor 

 of the hypothesis that the nutritive yolk acts as a tem- 

 porary dam to a system of nuclear actions and reactions 

 not of itself of equilibrium, is that these half embryos can 

 be obtained only from those eggs in which, as in the 

 Amphibia and Ctenophera, the nutritive yolk is abundant. 

 Whereas in the case of animals whose eggs are poor in 

 nutritive yolk, and present cleavage cells that are almost 

 identical, as in the Echinodermata and Ascidiae, either a 

 complete organism develops at once from the isolated 

 blastomere or postgeneration appears very early. 37 



Another fact which speaks in favor of this hypothesis 

 is that toward the end of a spawning period when the 

 vitality of the egg, and consequently also of the blasto- 

 meric nuclei, is lower, the formation of pure half 

 embryos is much easier to effect. 38 



While thus, after exclusion of pre formation theories 

 one can say that the formation of half embryos is a direct 

 proof of the hypothesis of a centroepigenesis with its 

 ramifying, independent correlation networks, this hypoth- 



36 Wilhelm Roux : Uber die kunstliche Hervorbringung ,,halber" 

 Embryonen etc. Virchows Archiv, Bd. 114. Oct. 1888. P. 132. 

 Gesamm. Abhandl. Zw. Bd. P. 442. 



37 Wilhelm Roux: Uber das entwicklungsmechanische Vermogen 

 der beiden ersten Furchungszellen des Eies. Verhandlungen der 

 Anat. Gesellsch. Wien, 1892. P. 5556. Gesamm. Abhandl. Zw. 

 Bd. P. 809-^810. 



38 Wilhelm Roux: Die Methoden zur Hervorbringung halber 

 Froschembryonen und zum Nachweis der Beziehung der ersten 

 Furchungsebenen des Froscheies zur Medianebene des Embryo. 

 Anatomischer Anzeiger, Bd. IX. February 1894. P. 257. Gesamm. 

 Abhandl. Zw. Bd. P. 954. 



