1893,] Embryology. 289 
This multiplication of the entoblastic cells is not equal on all sides, 
however, but is most frequent on the dorsal side of the invagination ; 
so that the ectoblast becomes invaginated itself at this point and push- 
ing the entoblast before it finally forms the roof of the cavity, while, the 
sides and floor are lined by entoblast. At the same time the margin 
grows backward and the gastrula mouth gradually closes. In this 
invagination, then we have two distinct processes: first, the invagina- 
tion of the entoblastic cells from which the gut is formed, (it is a palin- 
genetic process—the gastrulation); second, the invagination of the 
dorsal ectoblastic cells ; this is to be regarded as a coenogenetic process, 
that has nothing to go with gastrulation, but is preparatory to the 
formation of the chorda and the mesoblast. 
Lwoff was unable to find the pole cells of the mesoblast described by 
Hatschek, and it is interesting to note that more recent observations 
by E. B. Wilson have had the same result. Lwoff points out, more- 
over, that even if these cells did exist they could have no connection 
with the mesoblastic bands; the latter are upon the opposite side from 
the position assigned to the former. 
The multiplication of the cells in the medullary plate causes it to 
fold inward along the median, and it pushes before it the dorsal wall of 
the archenteron, leaving a mesoblastic fold on each side. The mesoblast 
is purely passive in its evagination. The inner half of each fold is 
composed of ectoblast cells from the roof of the gastrula cavity, and 
the outer half of entoblast from the side. 
The lumen disappears in each mesoblastic segment after it has been 
constricted off from the general fold. Afterward the true mesoblastic 
cavity, which is to become the body-cavity, is formed by the separation 
of the cells in the process of their growth. The body-cavity, therefore, 
is not a true enterocoel. 
The chorda is formed from what is left of the ectoblast in the gas- 
trula cavity, aided, perhaps, by the entoblast at the anterior end. The 
chordal plate becomes folded outward, and the two sides of the fold 
are pushed together by the entoblastic cells that at this stage are 
multiplying rapidly to form the dorsal wall of the gut. 
P. B. 
Epigenesis.—lIn an interesting review of the history of Evolution 
versus Epigenesis, Prof. C. Hertwig’ contributes a few experiments 
upon the eggs of Triton, to those of Chabry, Fiedler, Driesch and 
others all tending to overthrow the position occupied by Roux and 
35Entwichlungs-Theorieen, Berlin, 1892. 
