ON StHE MODES, ‘OF DEVELOPMENT OF THE 
MESODERM AND MESENCHYM, WITH REF- 
ERENCE, TOs Tik, SUPPOSED HOMOLOGS 
On thr BODY CAVITIES: 
THOS Ee MONTGOMERY, JiR, PHD: 
MESODERM is the term broadly applied to that embryonic cell 
mass situated between the primitive ecto- and entoderm. The 
mesoderm, z.¢., mesodermal cells, and not the extracellular gel- 
atinous substance secreted in the archicoel by the ecto- and 
entodermic cells, is derived either from one of these two layers 
or from both of them. It is derived (1) from the ectoderm in 
the Anthozoa, Porifera; (2) from the entoderm in Ctenophora, 
Polycladidea, Nemertini (in part), Nematodea, Annelida, Hir- 
udinea (?), Sipunculacea, Chaetognatha, Balanoglossus, Echin- 
odermata, Copepoda, Insecta (?), Mollusca, Phoronis, Ectoprocta, 
Brachiopoda, Entoprocta (?), Tunicata, Amphioxus; (3) from 
both ecto- and entoderm: in the Nemertini (Desor’s type), also 
perhaps in the Rotatoria and many of the Arthropoda (except 
Copepoda, Insecta). The origin of the mesoderm from the 
entoderm is thus the most usual. 
The mesoderm may arise from the entoderm in three ways : 
(1) by multipolar delamination ; (2) by unipolar delamination ; 
(3) in the form of epithelial diverticula. 
(1) The multipolar delamination, the so-called mesenchym 
production (Mesenchymbildung), consists in the separation of 
cells from the entodermal layer at various points on the inner 
surface of the latter, which become situated in the archicoel, in 
the gelatinous substance secreted by both ecto- and entoderm. 
Such mesenchym production is typical for the Nemertini, Ech- 
inodermata, and Ectoprocta. 
(2) The unipolar delamination of the mesoderm from the 
entoderm takes place through the so-called pole-cells of the 
mesoderm, z.¢., one or two cells which divide off from the ento- 
derm at one pole of the latter, and by the cleavages of which 
