356 MONTGOMERY. [Wor Sane 
the mesodermic cell layer is produced. These pole-cells always 
lie in the archicoel. Such a production of the mesoderm is 
typical for the Annelida and Mollusca; further, in more or less 
modified form, for the Nematodea, Copepoda, and Entoprocta. 
(3) Finally, the mesoderm may be produced from the ento- 
derm, in the form of epithelial diverticula, or the so-called 
coelom sacks. Such coelom sacks may be (a) evaginations of 
the gastrocoel, or (b) invaginations into the gastrocoel. 
(a) The mesoderm is produced in the form of a coelom sack, 
which is an evagination of the entoderm, and the coelomic cav- 
ity of which is consequently a derivative of the gastrocoel: in 
the Echinodermata (the vasoperitoneal vesicle), Balanoglossus, 
Brachiopoda, Ascideae, and Amphioxus. 
(b) The mesoderm is produced in the form of a coelom sack, 
which is an invagination of the entoderm, and the coelomic 
cavity of which is therefore derived from the archicoel, in 
Sagitta and Phoronis. 
The three types of development of the mesoderm delineated 
above are not sharply distinct, since they differ from one 
another rather in degree than kind, and are connected by inter- 
mediate stages. Thus we read on p. 12 of Korschelt and 
Heider’s Lehrbuch: “So verschieden diese beiden Arten der 
Mesodermbildung (namlich Urmesodermzellen, Urdarmdiverti- 
keln) auch scheinen mogen, so lassen sie sich doch auf ein ein- 
heitliches Schema zuriickfiihren, wenn wir annehmen, dass im 
ersteren Falle die Mesodermdivertikeln frihzeitig (als Urmeso- 
dermzellen) den epithelialen Verband des Entoderms verlassen, 
wahrend im zweiten Falle die Mesodermzellmasse vorlaufig in 
dem epithelialen Verbande bleibt und erst spater durch die 
Divertikelbildung zur Lostrennung gebracht wird” (cf also 
p. 267). In fact, I would refer both the production of the 
pole-cells and of coelomic diverticula to the more primitive 
mode of formation of multipolar mesenchymatic migration. 
By the mesenchym cells remaining in contact as an epithelium, 
coelomic diverticula are produced, which are bipolar but multi- 
cellular in origin and character. In the case of the two 
““Urmesodermzellen,” I would explain, in agreement with 
KKorschelt and Heider, the two pole-cells as two mesenchym 
