No. 3.] ZHE EMBRYOLOGY OF THE EARTHWORM. 389 
of the primitive blastopore. They are from the outset united 
in the middle line behind the posterior lip of the blastopore, 
but remain separate in front until the establishment of the 
mouth, when they extend forward, join in the median dorsal line, 
and thus form a complete longitudinal ring surrounding the 
region of the primitive blastopore. 
(2) The entire mesoblast is derived from a pair of primary 
mesoblasts or teloblasts that lie at the posterior ends of the 
germ-bands, and no mesoblastic elements arise from the ecto- 
blast overlying the germ-bands. The primary mesoblasts are 
differentiated in the course of the cleavage and are pushed into 
the segmentation-cavity some time before the beginning of 
gastrulation, in the manner described by Kleinenberg (No. 28). 
The cells formed by the continued proliferation of these primary 
mesoblasts are at a very early period differentiated into two 
groups which, though having a common origin and remaining 
in continuity, may conveniently be designated by different 
terms. 
The cells of the first group have histologically the character 
of mesothelium, form the mesoblastic parts of the germ-bands in 
the trunk region, and enclose the paired ccelomic cavities. They 
may therefore be called collectively the ¢runk-mesoblast. The 
cells of the second group arise by migration from the dorsal and 
anterior parts of the germ-bands, and may therefore be called 
the migratory mesoblast. They have histologically the character 
of mesenchyme, and form a nearly complete investment of the 
body in the trunk region, but also extend forward to form the 
cephalic mesoblast of the praoral lobe or prostomium. [It 
should be clearly understood that this distinction is made solely 
for the sake of convenient description, and that all the cells of 
both groups arise from the two primary mesoblasts and from no 
other source. ] 
(3) When fully established the germ-bands consist of three 
strata of cells, as in Hirudinea, namely: (2) an outer stratum 
(ectoblast), one cell in thickness, which arises directly from the 
original outer layer of the gastrula, and persists as the hypoder- 
mis; (4) an inner stratum (mesoblast) consisting of granular 
cells derived from the two primary mesoblasts; from it arise 
the muscles, dissepiments, blood-vessels, peritoneal epithelium, 
reproductive organs, and the inner part of the nephridia; (¢) a 
