528 MEMOIRS OF THE NATIONAL ACADEMY OP SCIENCES. 



Plate XXXIII. 



Pigs. 61, 62, 68, 69. Transverse sections of embryo in stage shown in Pig. 58, PI. xxxii. Pig. 61 

 cuts the thoracic-abdominal plate, and Pigs. 68 and 69 involve the optic disks. Pri- 

 mary yolk cells (P. Y. C. Pig. 69) are still plainly distinguishable. xll5. 



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

 in the invaginate area represent the mother cells of both mesoderm and eudoderra. 

 The yolk ball or secondary yolk segment is characteristic of this stage. x291. 



Pigs. 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 Pig. 64. x291. 



REFERENCE LETTERS. 



Ab. P., veutral plate. 



Ch., eggshell. 



Ep., ectoderm. 

 ^^ Ig., invaginate cavity. 



^ L. Cd., lateral ventral cord. 



0. D., optic disk. 



P. y. C, primary yolk cells. 



Sep., yolk cleavage plane. 



S. V. ' ., iu-wanderiug cells derived from ventral plate. 



T. B., yolk ball. 



