OLIGONEPHRIDIA 



275 



is retained until after blastokinesis, when the embryo comes again to the 

 surface and the yolk lies in the developing mid-gut lumen. 



The formation of the two layers of the germ band does not take place 

 simultaneously; nevertheless, it is to be regarded as a single step in the 

 development of the embryo. The outer layer constitutes the ectoderm; 

 the inner one, the meso- and entoderm. The vitellophags are to be 

 regarded as early segregated, abortive entodermal cells. Following the 

 appearance of the inner layer there are differentiated from this layer 

 mesenteron rudiments, two cell masses from which the mid-gut hning will 

 be formed. 



Soon after the three germ layers have been differentiated, the germ 

 band attains its maximum length, and the amniotic folds (Fig. 203 A, amf) 



mds 



Fig. 207. — Pyrrhocoris. Cross section of germ band. Differentiation of middle strand 

 {mds). {am) Amnion, {ect) Ectoderm, {il) Inner layer, {mds) Middle strand. 

 Paracytes. {ser) Serosa. 



{par) 



close over the orifice of invagination. Even before the germ band has 

 completely invaginated, the stomodaeum and then the proctodaeum 

 become evident (Figs. 203 A, B,stoni, prod). The amniotic cavity at the 

 edges is shghtly expanded segmentally (Fig. 205C,am. cav). In conse- 

 quence of this the lateral parts of the germ band together with the inner 

 layer are bent. Before the formation of the other organs the ectoderm 

 gives rise to a median strip on each side of which four longitudinal rows 

 of neuroblasts are formed. Tracheal invaginations occur at this time, 

 and near these the oenocytes arise. Four evaginations from the tip of the 

 proctodaeum, two lateral and two ventral, represent the rudiments of the 

 Malpighian tubules. As soon as the middle ectodermal strip is formed, 

 the cells of the inner layer that he above it break down except at the 

 extremities where these cells later are largely utilized for the formation 

 of the mid-gut. 



The inner layer now consists of two uninterrupted longitudinal 

 strands, one on each side of the median line. The inner layer lines the 



