538 MEMOIKS OF THE NATIONAL ACADEMY OF SCIENCES. 



Plate XXXVIII. 



(Stage VI.) 



Figs. 9(5-97. Longitudinal sections through the embryo shown in. Fig. 9.3. The more lateral of the 

 two, Fig. 96, cuts the middle of the optic disk, and shows the large cells of the prolif- 

 erating area, one of which is caught in the act of dividing. Fig. 97 cuts the inner 

 portion of the optic disk and the ventral plate on a level with the budding mandible 

 (Md,). x295. 



Figs. 9.S-100. Longitudinal serial sections through an embryo six hours older, from the same 

 batch of eggs. The mouth (Fig. 98, Std.) has already appeared. The mesoblast, 

 formed chiefly from wandering cells, is well established on either side of the middle 

 line of the body, and is well seen under the folds of the appendages (Fig. 100, Mes.) 

 into which it extends. The mesoblast represented by the lower layers of the ventral 

 plate is still being increased by the migration of cells from the surface of this plate, 

 as is indicated by cell ec, which is interpreted as a superficial cell about to migrate 

 (in Fig. 98). In Fig. 100 a cell at the surface of the oi)tic disk is in the act of delami- 

 uating. Large numbers of degenerating cells and their products are now encountered 

 (S.C.,s.). X29.5. 



REFERENCE LETTERS. 



A. (/), bud of tirst autenua. 



A. {II), bud of second antenna. 



Ab., Al. P., Ad. F., ventral plate. 



App., area of appendages. 



C. M., proliferating area of optic disk. 



fc, ec.', migrating and dividing cells at surface of ventral plate. 



Eel., Ep., ectoderm. 



.yd., rudiment of mandible. 



.Mes., mesoderm. 



0. C, optic ganglion. 



0. D., optic disk. 



«., s.'"^, products of degenerating chromatin. 



S. C, S. C.'~-, cells in various stages of degeneration. 



St. A., sternal area. 



Std., stomodaium. 



T. C(i. , Transverse sheet of ectoderm uniting optic disks. 



¥., Y. S., yolk spheres. 



