528 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



PLATE XXXIII. 



FIGS, 61, 62, 68, 69. Transverse sections of embryo in stage shown in Fig. 58, PI. xxxn. Fig. 61 

 cuts the thoracic-abdominal plate, and Figs. 68 and 09 involve the optic disks. Pri- 

 mary yolk cells (P. Y. C.. Fig. 69) are still plainly distinguishable. xl!5. 



FIG. 63. Portion of median longitudinal section of the same stage. The larger and clearer nuclei 

 in the invagiuate area represent the mother cells of both mesoderm and endoderra. 

 The yolk ball or secondary yolk segment is characteristic of this stage, x 291. 



FIGS. 64-67. Consecutive transverse sections of left optic disk of same stage before any thicken- 

 ing has occurred. The-most anterior section is represented in Fig. 64. x291. 



REFERENCE LETTERS. 



Ab. I'., veutral platf. 



Ch., eggshell. 



Ep., ectoderm. 



Ig., invagiuate cavity. 



L. Cd., lateral ventral cord. 



O. D., optic disk. 



P. >'. C., primary yolk cells. 



Sep., yolk cleavage plane. 



S. V. ' ., in-wandering cells derived from ventral plate. 



T. /(., yolk ball. 



