526 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Plate XXXI I. 



Figs. 50, 59, 60. Longitiidiual serial sections through the entire euibryo iu the stage shown iu Fig. 

 58. Fig. .59 is luediau. The primary yolk cells (P. Y. C, Fig. 60) can still be distin- 

 guished from the wandering cells derived from the invagination {S. T. C, Fig. 60). 

 In Fig. .59 a inimary yolk cell (P. Y. C.') is in the metakinetic stage of division. 

 Traces of tlie iniinary 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 {(Jh.) is not removed it is seen to l)e considerably distended and to 

 have epiblastic cells sticking to rt. showing the close adherence which normally exists 

 between the egg ineml)ranes and the egg. xll5. 



Fig. 57. Section through egg, cutting germinal disk just before invagination. Twenty anil one- 

 half hours older tiian yolk-pyramid stage seen in Fig. 15. x73. 



Fig. .58. Surface view of embryo after the uppearauce of the thorMcic-alidominal plate and the o])tic 

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

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



EEFKRENCE LETTERS. 



Ab. P., ventral plate. 



Ch., eggshell. 



Ep., Ep.', ectoderm. 



(t. T>., germinal disk. 



Ig., pit of invagination. 



L. Cd., lateral ventral bands. , 



0. D., optic disK. 



P. M., wandering cells, seen below surface, coiiiiii}; ofl' from ventral plate. 



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



Sep., yolk cleavage plane. 



S. r. C, S. Y. C, -wandering cells derived from the luvaginate cells and their products. 



T. Cd., cell area uniting optic disks. 



Y. B., yolk ball. 



Y. C, primary yolk cell. 



