540 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



PL AH; XXXIX. 

 (Stage, VI.) 



FIGS. 101-105. Serial longitudinal sections of early nauplius embryo, twelve and one-half hours 

 older than that represented by Figs. 08-100, PI. xxxvm, and eighteen and one half 

 hours older than the, stage represented in Fig. 93, PI. xxxvu. The thoracico-abdotni- 

 nal fold or papilla is now forming, apparently by the ingrowth of the surface ectoblast 

 (Fig. 104, A1>. .). Fig. 102 is exceptionally favorable in showing the undoubted 

 delamination of two cells standing side by side at the surface of the optic disk (,.). 

 The common radial division of the ectoderm of the thoracico-abdominal region 

 and other parts is illustrated by the cell ec. } of the same section. Fig. 105 cuts 

 the straight tubular stomodseuui. x295. 



FIG. loti. Longitudinal median section of embryo several hours older than the last. A deep, narrow, 

 transverse furrow (Ab. G.) now abruptly separates tin- thoracico-abdominal papilla 

 from the. sternal area lying between it and the stomodicum. x-91. 



FIG-. 107. Transverse section through the optic disks, from same stage. Cell delamination in this 

 region is again met with. x291. 



FIG. 108. Longitudinal lateral section .through entire egg, showing the distribution of wandering 

 cells, and the relations of the embryo to the ovum. The eggshell is unnaturally dis- 

 tended. An inner molted membrane is present, as is better shown in Fig. 106 and 

 Fig. 104, M/>. 



In Fig. 108 a large cell is seen at the surface, and below this a large, cell followed 

 by a row of similar cells. The first two cells possibly represent a budding ectoblast 

 and mesoblast, and the rows of cells at the surface and below it are possibly derived 

 from them, x 115. 



REFERENCE LETTERS. 







A. (I), Inul of first antenna. 



.1. (II), Inul of second antenna. 



Ah., thoracico-abdominal papilla. 



Ah. ('., transverse superficial furrow by which folil of Mm thoracico-abdominal process is formc'd. 



.1. )'. iS., products of degenerating chromatin. 



/>'. /., building zone. 



''/(., eggshell. 



' '. .I/., proliferating area of optic disk. 



ri. N., cells on ventral side, of yolk next to optic disks, probably representing mesoblasf derived from 



\\ andcring cells. 



re., ?<.', dividing ectoblastic cells. 

 /'c/., /-'/<, ectoderm. 



,>/.. wandering cell at surface behind thoracico-abdominal fold (Kig. UiH). 

 Mli., embryonic molt. 

 Mil., rudiment of mandible. 

 Mf*., mesoblast. 

 <>. !>., optic disk. 

 ". '.'.. optic, ganglion. 

 0. N. G., brain. 

 I'd., proctodicmn. 



N.. N. , products of degenerating chromatin. 

 A'. C'., ilegenerat.ing cells. 

 *>''. A., sternal area. 



*lll., Momod;i'lllll. 



).. 1. S., yolk. 



