564 MEMOIES OF THE NATIONAL ACADEMY OF SCIENCES. 



Plate LI. 



(Stage XL) 



Figs. 183-186. Continuation of series of transverse sections of embryo begun on Plate L. x 74. 



Fig. 187. Part of sagittal section of similar embryo, cutting eyestalk somewhat obliquely. The 

 specimen was depigmented in nitric acid. The distal retinular cells, occupying the 

 spaces (Pg. c.) between the peripheral ends of the cones, are not represented. x305. 



REFERENCE LETTERS. 



ac. J'., accessory pigment cells. 



nd. m., adductor of mandible. 



n. sa., superior abdominal arte'y. 



hg., branchiostegite. 



h. i\, blood vessel. 



0. . crystalline cone. 



eg., corneagen. 



cct., ectoderm. 



aid., endoderm. 



fg., foregut. 



B., heart. 



lit/., bypodermis. 



imb., intercepting mombrane. 



mg., midgut. ~ 



mg.^, posterior lobe of midgut. 



A/«., muscle of eyestalk. 



Mil. e., extensor muscles of abdomen. 



Mil./., llexor muscles of abdomen. 



o. c. m., oesophageal commissure. 



Pg. C, position of distal retinular cells. 



P.I., pericardial sinus, 



Ptii., proximal retinular cells. 



Ulit.', nuclei of proximal retinular cells. 



3, 4, ganglia of eyestalk. 



