552 MEMOIK8 OF THE NATIONAI, ACADEMY OF SCIENCES. 



Platk XLY. 

 (Stage VIII.) 



Fig. 130. Lateral loagitudiual 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 14.5. Fig. 136 is to be com[)ared wiMi tlie sliglitly older embryo in Fig. 129. 

 Blood cells {B. (J.) and other wandering ceils are liere seen settling down upon the 

 body wall. A wandering cell is also seen nearly in contact wit li t lie optic ganglion, x 241. 



Fia. 137. Transverse section of embryo in same jdiase, just Iteliind tlie level of the first antennae, 

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



Figs. 138, 139. Serial longitudinal sections through embryo in Stage VIII. Fig. 138 should be 

 compared with Figs. 136 and ll'!). All the ganglia of the nervous system, at least as 

 far back as the eighteenth segment, are marked at the surface by deej) 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 thoracicoabdominal 

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

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

 •X241. 



Figs. 140-14.'i. 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 (MS., ws.') 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 ¥\g. 34. x 241. 



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 {T)p.) — which is characteristic of a later stage 

 (Fig. 153, Dp.). The strictly superficial cells of the dorsal plate are probablj' 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 

 thoracicoabdominal fold. The wandering cells which appear in this part were taken 

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



REFERENCE LETTERS. 



^' 



A. I, lirst antenna. 



A. II, second antenna. 



an., lower margin of optic lobe. 



A. a. a., superior abdominal artery. 



li. C, blood corpuscle. 



b.m., basement membr.ane. 



cA., eggshell. 



Dp., dorsal plate. 



End., endoderm. 



O. IV-XniI, segmental ganglia. 



Gl., gangliogen. 



H., heart. 



hd., hypodermis. 



Hg., hindgut. 



mes., mesoblast. 



jiio., month. 



ms., ma.', wandering cells at surface. 



O. L., optic lobe. 



Rt., retinogeii. 



Std., stomodicum. 



Th. db., thoracic-abdominal fold. 



j/.c, wandering cells. 



