1156 The American Naturalist. ' [December, 
simply mixed with one another. But where its rudiment is rep- 
resented by a single pair of pole-cells these must be regarded as 
blastomeres precociously removed into the blastoccel, and contain- 
ing in themselves the future sources of the primary and secondary 
mesoderm, still unseparated. Since the mesenchym has here and 
there its ewn pole-cells —for we must regard as such the nephro- 
blasts of Whitman and Wilson, and the lateral teloblasts of some 
Hirudinea and Oligochzta according to the later observations of 
Berg upon the Lumbricide,—those mesoderm pole-cells that 
later give rise to both primary and secondary mesoderm may 
have been at one time blastomeres, which by division gave rise to 
the pole-cells of the ccelomic mesoderm, as well as to mesenchym 
pole-cells, but which later no longer divided. 
The above very sketchy presentation of my views of the 
mesoderm may be summed up as follows: As in Kleinenberg's 
theory, the entire mesoderm is not to be regarded as a uniform 
structure of equal significance with the two primary germ layers, 
but as a combination of * Anlagen" of very diverse organs that 
arose at one time entirely independently of one another. But 
an important element of it, the so-called secondary coelomic 
mesoderm, or the genito-peritoneal embryonic tissue, as I would 
call it, has, as contrasted with the embryonic mesenchym, the 
significance of a primitive organ, —namely, of an ancestral sexual 
or gonad tissue that arose from the primary sexual cells, belong- 
ing to neither germ layer, in the oldest many-celled animals. 
Warsaw, April, 1890. 
