524 MEMOIKS OF THE NATIONAL ACADEMY OF SCIENCES. 



PLATE XXXI. 



FIGS. 49-55. Serial transverse sections through the embryo in the iuvagination stage. In the 

 most anterior section the germinal area (O. Z>.) is traversed, and in Fig. 53 the shal- 

 low depression in the middle of the invaginate area is cut through. In Fig. 54 (G. 

 D.) we see the forward extension of the invagiuate cells and the first trace of the 

 thoracic-abdominal plate. The distinction between the primary yolk cells (Figs. 49, 

 52, 53-55) and the invagiuate wandering cells (b, 6 2 " 1 '', Figs. 5H-54) and their product, 

 which now begin their migrations, is very plainly shown. In Fig. 50, which cuts 

 the shallow pit at the surface of the invagiuate area, we see the amosboid cells with 

 large granular nuclei making their way from the bottom of the pit into the depths 

 of the yolk. Figs. 49, 52-55, xl!5. -Figs. 50, 51, x291. 



REFERENCE LETTERS. 



b, b'^b", in-wanderiug cells derived from the invaginate cells aud their products. 



Ch., egg capsule or shell. 



Ep., ectoblast. 



G. D., embryonic area. 



/. C., iuvaginate cell. 



Ig., pit formed by the in vagi nation. 



Y. B., yolk ball. 



P. Y. C., primary yolk cell. 



Y. S., yolk sphere. 



