530 MEMO1RF OF THE NATIONAL ACADEMY OF SCIENCES. 



PLAIK XXXIV. 

 (Stage IV.) 



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

 72. In Fig. 73 the plane 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, xllo. 



FIG. 72. Surface view of embryo in Stage iv. Rudiments of the mandibles and first pair of an teuuse 

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

 the large si/e of the nuclei. From them and their products the optic ganglion takes 

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

 tally cut away, xli'.ll. 



REFERENCE LETTERS. 



.I. (I.), proliferating center af firs! antenna. 



Ah. P., ventral plate. 



C. jU., proliferating area of optic dink. 



j)., ectoderm. 



/,. Cd., lateral ventral cord. 



MA., proliferating center of mandible. 



O. D., optic disk. 



T. Cd., transverse cord uniting optic disks. 



Y. A'., yolk ball. 



Y. C., T. C.', wandering cells. 



