248 



INVERTEBRATA 



CHAP. 



At the same time, on the ventral side of the egg two slightly 

 curved longitudinal folds make their appearance, and divide the 



ser 



am 



germ 



term. 



FIG. 196. Diagrams to show the relations sustained to one another of amnion, serosa, and 

 embryonic area in three successive stages of the development of Donacia crassipes. 

 (After Hirschler.) 



A, youngest stage. B, intermediate stage. C, oldest stage. In each pair of figures the left-hand 

 one represents the ventral view, the right-hand one the dorsal view, am, amniotic area; errib, em- 

 bryonic area ; gast.gr, the limits of the gastral groove ; germ, mass of primitive germ cells ; ser, serosa. 



blastoderm into a median and two lateral plates. The median plate 

 gives rise to endoderm and mesoderm, whilst the lateral plates give 



