526 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



PLATK XXXII. 



* 



FIGS. .">(, 5!l, 60. Longitudinal serial sections through the entire embryo in the stage shown in Fig. 

 58. Fig. ~>9 is median. The primary yolk cells (P. Y. C., Fig. 60) can still be distin- 

 guished from the wandering cells derived from the invagination (*S*. Y. 6'., Fig. <><>) 

 In Fig. .">!> a primary yolk cell ( /'. Y. r. 1 ) is in the metakinetie stage of division. 

 Traces of the primary segmentation of the yolk are still present, and the secondary 

 yolk segmentation is very marked in the neighborhood of the wandering cells. 

 Where the .shell (Ch.) is not removed it is seen to be considerably distended and to 

 have, epiblastic cells sticking to it. showing the ('lose adherence which normally exists 

 between the egg membranes rind the egg. x 11">. 



Fid. 57. Section through egg, cutting germinal disk just before invagination. Twenty and one- 

 half hours older than yolk-pyramid stage seen in Fig. 1">. x 73. 



Fi<;. -"iS. Surface view of embryo after the appearance of the thoracic-abdominal plate and the optic 

 disks. The shallow depression which marked the invaginate area has disappeared. 

 Its approximate position is indicated by ///. Compare /</., Fig. 59. x291. 



REFERENCE LETTERS. 



All. I'., ventral plate. 

 Cli., eggshell. 



P.p., J':j>.', eCtoderMl. 



fr. /)., germinal disk. 



Iy., pit of invagination. 



L. I'd., lateral ventral hands. 



O. !>., optic disK. 



P. M., wandering rolls, wen In-low surfarr, mining oil' from ventral )>late. 



P. Y. C., P. T. C.\ primary yolk cells. 



j>. , yolk cleavage plane. 



.S. F. r., ,S. Y. C. 1 , wandering cells derived from the invaginate cells and tlieir p 



T. <'ii.. cell area uniting optic disks. 



T. /,'., yolk hall. 



Y. C., primary yolk cell. 



