454 WILSON. 
EXPLANATION OF PLATE XVIII. 
(Nos. 47 to 52 and 57, 58 from Z. fetidus, Nos. 53, 54, L. communis, Nos. 55, 56 
from an unknown fresh-water oligochzete, the embryos of which are in all essential 
features like those of Lumbricus.) 
Fic. 47. Right ventro-lateral view of an embryo at the earliest period in which the 
mesoblasts and nephroblasts are distinguishable. Right germ-band in surface view, 
left in optical section. X 300. 
Fic. 48. Right lateral view of embryo with completely established germ-bands, 
showing position of the teloblasts, cell-rows, etc. xn70) 
Fic. 49. Ventral view of the same embryo. The germ-bands wholly separate be- 
low. (Outlines with camera. Cells diagrammatic.) 
Fic. 50. Right lateral view of older living embryo, showing the stomodzum, ar- 
chenteron, the ccelomic and prostomial cavities, developing nephridia, and the dissepi- 
ments. The most anterior of these (¢!) form the ventral and later also the posterior 
boundary of the prostomial cavity. x 170; 
Fic. 51. Left lateral view of embryo in the middle period of development. The 
anterior teloblasts have broken up (see Fig. 63 for diagram of the germ-bands); the 
primary mesoblasts remain. The dorsal limit of the mesoblast is marked by the lat- 
eral vessel (/. v.), which ultimately coalesces with its fellow to form the dorsal vessel. 
<0: 
Fic. 52. Transverse (actual) section through the embryo shown in Fig. 37. The 
mesoblastic bands are quite distinct from the ectoblast through their whole extent, 
and are now widely separated in front. X 300. 
Fics. 53 AND 54. Transverse (slightly oblique) section through an embryo at a 
somewhat earlier stage than Figs. 48, 49. No. 54 (the more posterior) passes through 
the primary mesoblasts (which appear on the apparent dorsal side owing to the great 
curvature of the bands) and the two nephroblasts of the left side. No. 53 cuts the 
left mesoblast (/Vé.), the left (double) nephric cord, and one of the right nephro- 
blasts. All these teloblasts lie at the surface. X 300. 
Fics. 55 AND 56. Two consecutive horizontal sections through the anterior region 
of an embryo in which the prostomial cavity has just appeared. In No. §5 (more 
dorsal) the extreme anterior part of the mesoblast appears, and the bridge of neural 
tissue (apparently fused with the ectoblast) that joins the two cephalic ganglia. In 
No. 56 the prostomial cavity appears, lying between the cephalic ganglia. In the 
following section this cavity disappears, and the dorsal wall of the stomodzum 
appears. eR 
Fics. 57 AND 58. Two transverse sections through the anterior part of an older 
embryo. No. 57 (the more anterior) shows the prostomial cavity and the dorsal wall 
of the stomodeum. No. 58 (two sections further down) shows the anterior corners 
of the anterior pair of ccelomic cavities, with the prostomial cavity between them. 
X 300. 
(All the figures with the camera.) 
