552 MEMOIKS OF THK NATIONAL ACADEMY OF SCIENCES. 



PLAIT, XLV. 

 (Stage VIII.) 



FIG. 136. Lateral longitudinal section of embryo in stage intermediate between YII and VIII, 

 represented in surface view in Fig. 110. To this phase also belong Figs. 137, 144. 

 and 145. Fig. l.'ili is to be compared with the slightly older embryo in Fig. 129. 

 Blood cells (B. V.) and other wandering cells are here seen .settling down upon the 

 body wall. A wandering cell is also seen nearly in contact witli the optic ganglion, x-41. 



FIG. 137. Transverse section of embryo in same phase, just behind the lev,el of the first antennae, 

 showing the relations of the wandering colls at this period to the embryo and egg. x 61. 



FIGS. 1, '58, 139. Serial longitudinal sections through embryo in Stage VIII. Fig. 138 should be 

 compared with Figs. 136 and 129. All the ganglia of the nervous system, at least as 

 far back as the eighteenth segment, are marked at the surface by deep constric- 

 tions. The ganglia of the nineteenth and twentieth segments are less distinct. The 

 ganglia of the eleventh segment lie in the angle made by the thoracico-abdomiual 

 flexure, Wandering cells occur in the yolk, lint are less abundant, and the products 

 of cell degeneration, which enter into the general nutrition, have mostly disappeared. 

 X241. 



FIGS. 140-143. Parts of sections taken at various points on the surface of the egg (series to which 

 Figs. 136, 137, 144, 145, belong), remote from the embryo, to show the role of certain 

 wandering cells which reach the surface and represent mesoblast. In Fig. 140 two 

 cells (ins., ms. 1 ) are -partially flattened against the surface, but here, as in Fig. 142, 

 the wandering cell ms. is clearly distinguishable from the spindle-shaped ectoderm 

 cell on the left. Compare Fig. 34. x241. 



FIGS 144-145. Serial longitudinal sections through the embryo and entire egg to show the distri- 

 bution of the wandering cells. Certain wandering cells not yet flush with the surface, 

 enter into an organ the dorsal plate (TJp.) which is characteristic of a later stage 

 (Fig. 153, Dp.). The strictly superficial cells of the dorsal plate are probably in all 

 cases ectoblast, and some of the wandering cells degenerate before they reach the 

 surface ectoblast. 



There seems to be a general dispersal of wandering cells from the vicinity of the 

 thoracico-abdominal fold. The wandering cells which appear in this part were taken 

 from four consecutive sections, including that represented in the drawing. x61. 



REFERENCE LETTERS. 



A. I, lirst antenna. 



A. II, second antenna. 



., lower margin of optic lobe. 



A.s. a., superior abdominal artery. 



IS. (.'. , blood corpuscle. 



b. HI., basement membrane. 



oh., eggshell. 



Dp., dorsal plate. 



End., endoderm. 



G. IV-SriII, segmental ganglia. 



Gl., gaugliogen. 



B., hc:iit. 



hil., liypodcrniis. 



ffff., hindgut. 



ie., mesoblast. 



mo., month. 



ms., ms. 1 , wandering cells at surface. 



0. L., optic lobe. 



I{t., retinogen. 



Std., stouiodit'iiiii. 



T/i.ab., thoracic-abdominal fold. 



i/. a., wandering cells. 



