540 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Plate XXXIX. 

 (Stage VI.) 



Figs. 101-1j05. Serial longitudinal sections of early nauplius embryo, twelve and one-half hours 

 older thai) that represented by Figs. 98-100, PI. xxxviii, and eighteen and oue-balf 

 hours older than the stage represented in Fig. 93, PI. xxxvii. The thoracicoabdonii- 

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

 (Fig. 104, Ab. C). Fig. 102 is exceptionally favorai)]e in showing the nn(lotil)ted 

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

 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 stomodaeum. x 295. 



Fiti. 106. Longi tudinal median section of embryo several hours older than the last. A deep, narrow, 

 transverse furrow {Ab. G.) now abruptly separates the thoracico-abdominal papilla 

 from the sternal area lying between it and the stomodieum. x291. 



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 molt-ed membrane is present, as is better shown in Fig. lOG and 

 Fig. 104, Mb. 



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. (/), ')iiil of tirst auteuua. 



^. (i/), bud of second antenna. 



Ai., thoracico-abdominal papilla. 



Ah. C, transverse superficial furrow by which fold of the thoracico-abdominal )>roee88 is formed. 



A. y. S., products of degenerating chromatin. 



B. '/.., budding zone. 

 CA., eggshell. 



C. M., proliferating area of optic disk. 



Ci. S., cells on ventral side of yolk next to optic disks, probably representing mesoblast derived from 



wandering cells. 

 ec, ec.', dividing ectoblastic cells. 

 Ect., Ep., ectoderm. 



M.. wandering cell at surface behind thoracico-abdominal fold (Fig. 108). 

 Mh., embryonic molt. , 

 Aid., rndiment of mandible. 

 Mea., mesoblast. 

 0. D., optic disk. 

 0. G., optic ganglion. 

 O. S. G., brain. 

 Pd., proctodaMiin. 



S., S.'-"-, products (it degenerating chromatin. 

 ^S. C, degenerating cells. 

 St. A., sternal aiea. 

 Std., stomodwum. 

 F., F. S., yolk. 



