524 MEMOIKS OF THE NATIONAL ACADEMY OF iSClENCES. 



Plate XXXI. 



Pigs. 49-55. Serial transverse sections througli the embryo in the invagination stage. In the 

 most anterior section the germinal area (G. D.) is traversed, and in Fig. 53 the shal- 

 low depression in the middle of the invaginate area is cut through. In Fig. 54 [Q. 

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

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

 52, 53-55) and the invaginate wandering cells (b, 6", Figs. 52-54) and their i)roduct, 

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

 the shallow pit at the surface of the invaginate area, we see the amoeboid cells with 

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

 of the yolk. Figs. 49, 52-55, x 115. Figs. 50, 51, x 291. 



REFERENCE LETTERS. 



b, b'^ly', in-wandering cells derived from the invaginate cells and their products. 



Ch., egg capsule or .shell. 



Ep., ectoblaat. 



fi. Z)., embryonic area. 



/. C, invaginate cell. 



Ig., pit formed by the invagination. 



F. B., yolk ball. 



P. Y. C, primary yolk cell. 



F, S., yolk sphere. 



