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Prof. W. Salensky on the 
stages of development gives rise to the atrial tubes and the 
nerve-ganglion. The bulk of the embryonic mass consists 
of the undifferentiated elements of the two other germinal 
layers, and may therefore be termed the meso-endoderm ; 
before this gives rise to the rudiments of various organs it 
undergoes a differentiation, resulting in its splitting into two 
germinal layers, the mesoderm and endoderm. The differen- 
tiation of the endoderm occurs tolerably late — not until after 
the formation of the coelomic cavities in the mesoderm. The 
formation of the coelomic spaces and their metamorphoses are 
what we now have to consider. 
If we examine sections from the nuclear mass, in 
which mesoderm and endoderm are represented by a still 
undifferentiated mass of cells, we at once notice in the interior 
of these sections several lacuna-like cavities which as yet 
have no connexion with one another. These cavities are the 
earliest rudiments of the subsequent coelomic spaces, which 
convert the solid mesoderm into two coelomic sacs. Whether 
or not these cavities are symmetrically arranged from the 
first I cannot decide with certainty, as I had at my disposal 
but few embryos in these stages. At any rate they appear 
to be symmetrically arranged in the following stage, in which 
the nuclear mass flattens out and assumes the form of a 
germinal disk. It is very probable that all the isolated 
cavities coalesce at this period, since the coelom is now no 
longer represented by several separate spaces, but by two 
large cavities lying one on each side of the longitudinal axis 
of the germinal disk. At much the same time as this 
important changes also take place in the germinal disk itself ; 
the lower surface of the latter recedes from the upper surface 
of the yolk, in consequence of which a space is left between 
the yolk and the germinal disk, which is subsequently trans- 
formed into the enteric cavity. The mutual relations of the 
two spaces, the enteric cavity and the coelom, can be deter- 
mined by means of sections ; and in successful ones we can 
clearly see that the two coelomic sacs open into the cavity of 
the intestine. In the median line between the two openings 
of the coelom, in the axial portion of the germinal disk, there 
projects into the enteric cavity a longitudinal ridge, which is 
likewise traversed by a canal. The opening of this canal I 
was not able to make out ; but with regard to the interpreta- 
tion of the two lateral openings of the coelomic sacs, their 
relation to the intestinal cavity points to the conclusion that 
we have in these openings the homologues of those described 
by van Beneden and Julin, through which the primitive 
enteric cavity communicates with the coelomic sacs. Although 
