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.%® 
- 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.?? 
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.*8 
While thus, after exclusion of preformation 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- 
*°Wilhelm Roux: Uber die kiinstliche Hervorbringung ,,halber“ 
Embryonen etc. Virchows Archiv, Bd. 114. Oct. 1868 P. 132. 
Gesamm. Abhandl. Zw. Bd. P. 442. 
*TWilhelm Roux: Uber das entwicklungsmechanische Vermégen 
der beiden ersten Furchungszellen des Eies. Verhandlungen der 
Anat. Gesellsch. Wien, 1892. P. 55—56. Gesamm. Abhandl. Zw. 
Bd. P. 809—8r1o. 
**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. 
