530 MEMOIEF OF THE NATIONAL ACADEMY OF SCIENCES. 



Platk XXXIV. 

 (Stage IV.) 



Figs. 70, 71, 73, 74. Longitudinal serial sections of the entire embryo in the stage shown in Fig. 

 72. lu Fig. 73 the i)lane of section is nearly median. The primary yolk cells are 

 now generally indistinguishable from the other wandering cells. Compare cut, Fig. 

 11, which shows the distribution of the wandering cells at this stage. xll5. 



Fig. 72. Surface view of embryo in Stage iv. Rudiments of the mandibles and first pair of autennse 

 are present. An area of cell ingrowth in the optic disks {C. M.) is characterized by 

 the large size of the nuclei. From them and their j)roducts the optic ganglion takes 

 its origin. Some of the surface cells on either side of the middle line were acciden- 

 tally cut away. x291. 



KKFKBENCE LBTTEKS. 



-). (/. ), proliferating center of first .auteuna. 



Ab. P., ventral plate. 



C. M., proliferating area of optic disk. 



Ep., ectoderm. 



L. Cd., lateral ventral cord. 



Md., prolifer.ating center of mandible. 



O. D., optic disk. 



T. Cd., transverse cord uniting optic disks. 



T. B., yolk ball. 



F. C, Y. C, wandering cells. 



