536 MEMOIRS OF THE NATIONAL ACADEMY OP SCIENCES. 



PLATE XXXYTT. 

 (Stages V-VI.) 



FIG. 92. Part of transverse sectiou, showing the structure of the keel-shaped ventral plate, aud 

 indicating the origin of mesoblast from the surface of the latter. x291. 



Fi<;. !>3. Surface view of embryo with buds of naupliar appendages. The intermediate area (St. A.) 

 is covered by a single layer of ectoderm. The invagination of the mouth has not 

 yet appeared. Some nuclei of cells which lie immediately below the surface, especially 

 in the thoracic abdominal plate region, are represented. x291. 



FIGS. 94-95. Transverse sections of embryo, belonging to the same series as Fig. 92, to illustrate 

 the structure of the thickening optic disks. Degenerating nuclear products (.', ..-, 

 Fig. 95) are present, and two cells are seen delaminating side by side in Fig. 95. 



REFERENCE LETTERS. 



A. (/), rudiment of first antenna. 



A. (II ), rudiment of second antenna. 



Ab. /'., ventral plate. 



''. M., proliferating area of optic ganglion. 



Kft., ectoderm. 



Md., rudiment of mandible. 



Mes., wandering cells (mesoblast) attached to ectoderm. 



O. G., rudiment of optic ganglion. 



O. D., optic disk. 



.', s. 2 , products of degenerating chromatin. 



St. A., sternal area. 



T. Cd., transverse cord uniting optic disks. 



T. C., wandering cell; Y. C.\ wandering cell degenerating. 



