THE 
AMERICAN NATURALIST 
Vor. XXIV. OCTOBER, 1890. 286. 
THE MESODERM AND THE CCELOM OF 
VERTEBRATES. 
BY CHARLES-SEDGWICK MINOT. 
HE morphology of the mesoderm is one of the most vexed 
questions of the day. Scarcely an embryologist can be 
found who has not published opinions on this question consider- 
ably at variance with those of most others. It has been main- 
tained that the mesoderm arises from the ectodernt; that it arises 
from the the entoderm, or from both; from neither, but from two 
special segmentation spheres; that it has a double origin, part 
coming from the blastoderm, part from the yolk; and even that 
there is no mesoderm. 
We now know positively that in all vertebrates there is a dis- 
tinct and unmistakable mesoderm, which spreads out from the 
primitive streak in all directions, and has distinctive histological 
characteristics. Two large cavities appear in this mesoderm on 
either side of the median axial line. The mesodermic cells which 
bound these two cavities assume an epithelial arrangement, and 
are designated as the mesothelium; the cavities constitute the 
celom ; the remainder of the mesoderm is known as the mesen- 
chyma, and corresponds to the embryonic connective tissue of 
older writers. The mesothelium at various points throws off 
cells, which are added to the mesenchyma. We have accordingly 
two distinct phases to study, viz., the origin of the mesoderm, 
and the differentiation of the mesenchyma and mesothelium, and 
together with the latter the formation of the ccelomatic cavities. 
