514 



R. W. Hegner 



in the peripheral layer, but collect about them a number of granules 

 (germ-cell determinants, Fig. ij, g c d) which they encounter in 

 this region and continue their migration until they are entirely 

 separated from the blastoderm. These are the primordial germ- 

 cells (Fig. 3, p^g c). The first change noticed in the blastoderm is a 

 crowding together of the cells on the ventral surface of the egg. 

 This results in the formation of a broad longitudinal band of closely 

 aggregated cells, the ventral plate. The edges of this plate are 



t.f— -4 



cap 



--t.aR 



7. 8. 



Fig. 7 Surface view of embryo described in Series C.B. 2, c. The embryo has begun to broaden 

 and shorten. 



Fig. 8 An embryo of C. bigsbyana in which the tail-fold is coincident with the posterior end of 

 the egg. cap. ~ cephalic appendage, p. = posterior, t.ap. = thoracic appendage, t.j. = tail-fold. 



thrown up into two folds; these spread out in the posterior region 

 extending to the end of the egg where they pass around the pri- 

 mordial germ-cells and meet on the dorsal surface. The ventral 

 plate now decreases both in length and in breadth and a longi- 

 tudinal concavity, the ventral groove, appears. The germ-band 

 can now be recognized; it covers the entire ventral surface of the 

 egg except a wedge-shaped area anterior to the groove (Fig. \y gh). 



