452 WILSON. 
EXPLANATION OF PLATE XVII. 
Lumbricus fetidus. 
Fics. 24 TO 30. Series of optical sections showing the early history of the meso- 
blast. Figs. 24, 26, 27 are side views; Figs. 29 and 30 horizontal views; Fig. 28 is 
an oblique lateral view of the same specimen shown in Figs. 27 and 29. The series 
shows the gradual enclosure of the primary mesoblasts within the segmentation cavity, 
the beginning of the mesoblastic cell-rows and the variation of the cleavage-pore, 
which is absent from Figs. 24, 25, and 30, but present in the others. The mesoblastic 
rows meet in front. 
Fics. 31 AND 32. Dorsal and lateral views of a blastula in which the flattening 
preparatory to invagination has begun. The layers are already clearly differentiated, 
there is a large segmentation cavity, and the mesoblastic rows still meet in front. 
Fic. 33. Dorsal view of a slightly later stage (cf Figs. 34, 39, 40, 45, 46), show- 
ing the entoblast, the ectoblast in optical section at the sides, the ectoblastic nuclei 
of the dorsal surface, and the primary mesoblasts. 
Fic. 34. Ventral view of the same embryo, showing the entoblast nuclei and the 
edge of the ectoblast. 
Fic. 35. Ventral view of early gastrula (the anterior extremity directed upwards), 
showing the ectoblast at the sides and below, the entoblast, and the extreme anterior 
end of the right mesoblastic row in optical section at 7. s. 
Fic. 36. Gastrula, ventral view, showing the blastopore, the anterior ends of the 
ectoblastic part of the germ-bands (ec.), and the nuclei of the flattened cells of the 
ventral ectoblast. 
Fic. 37. Corresponding view of another specimen in which the lateral infolding 
has advanced more rapidly than in the last, so as to give rise to a slit-like blastopore. 
(Compare Fig. 42. For actual section of this specimen, see Fig. 52.) 
Fic. 38. Lateral view of established gastrula in which the mesoblastic bands have 
met above the mouth. The left primary mesoblast (JZ) and mesoblastic band (ms.) 
shown. Archenteron and ectoblast in optical section. Limit of ectoblastic part of 
the germ-band indicated by the faintly drawn nuclei. (Compare Fig. 43.) 
Fic. 39. View of flat gastrula (shown in Fig. 33) from the right side, showing the 
ectoblast, entoblast, and one primary mesoblast in optical section, and also the ecto- 
blastic nuclei of the right side. (For actual sections of the same specimen, see Figs. 
45 and 46.) 
Fic. 40. The same specimen in transverse optical section, showing the mesoblastic 
bands (vs.) at the sides. 
Fic. 41. Surface view from the left side and below, of the specimen shown in Fig. 
36. Only the ectoblastic part of the germ-band is shown, except the primary meso- 
blast (JZ). Ectoblast in optical section at the sides. 
Fic. 42. Lateral view of gastrula at about the same stage as Fig. 37, showing the 
right mesoblastic band (superficial ectoblast not shown). 
Fic. 43. Anterior view of the embryo shown in Fig. 38 (slightly diagrammatic), 
to show the anterior union of the germ-bands above the mouth and their approxima- 
tion on its ventral side. 
Fic. 44. Dorsal view of the embryo shown in Fig. 42, showing the archenteron, 
the lateral ectoblast, the primary mesoblasts lying in contact in the middle line, and 
the two mesoblastic rows (77s.). 
Fics. 45 AND 46. Longitudinal (actual) sections from a complete series of the 
embryo shown in Figs. 33 and 39._ Fig. 45 is towards the side and shows the right 
mesoblastic band in nearly its whole extent. Fig. 46 is nearly in the median line, 
passing through one of the primary mesoblasts. ‘The segmentation cavity has disap- 
peared. The series shows beyond question that the mesoblastic bands are separate in 
front. No trace of the Schluckzellen can be seen. 
(Enlargement 300 diameters. All the figures with the camera excepting Nos. 32, 
39, and 40.) 
