OLIGONEPHRIDIA 



249 



eral nuclei by repeated division soon forming a primary epithelium 

 (blastoderm). The vitellophags meanwhile have also increased in num- 

 ber. The primary epithelium thickens ventrally, and at the anterior and 

 posterior ends within and close to the epithehum some small cells whose 

 nuclei stain more lightly than the epithelial cells constitute the entodermal 

 rudiments (Fig. 168, ent). In addition, at this time, the primordial germ 

 cells are found at the posterior end (Fig. 168, gc). Certain cells, which 

 wander into the central nutritive mass from the ectoderm, may be con- 

 sidered as forming a middle ento- 

 dermal mass. 



After the development of the 

 entoderm, the mesoderm and the 

 ectoderm begin to differentiate. 

 The sides of the ventral germ band 

 become thickened by rapid multi- 

 plication of cells 



7 



mp 



Fig. 169. — Archipsocus. Cross section. Fig. 170. — Archipsocus. Sagittal sec- 



{e7U) Entoderm. (Ip) Lateral plate, (mp) tion. (am) Amnion, {ent) Entoderm, (gc) 

 Median plate, (trc) Trophocyte. Germ cells, (ser) Serosa, iovt) Wall of 



ovariole. 



formation of a median plate and two lateral plates (Fig. 169, mp,lp). 

 Meanwhile, cells are proliferated along the entire length of the median 

 plate. The mesal edges of the lateral plates then grow toward each other 

 until they meet on the median line without the formation of a gastrular 

 furrow. The lateral plates form the ectoderm; the median plate, the 

 mesoderm. 



The germ band now becomes flexed (Fig. 170). Head and tail folds 

 appear which grow around the embryo to meet ventrally and thus com- 

 plete the amnion (am) and serosa (ser). 



Later, body segmentation and rudiments of the appendages appear. 

 The antennae, the mandibles, and the three thoracic appendages are first 

 conspicuous, followed later by the other mouth appendages. Ten distinct 

 segments are to be found in the abdomen. The stomodaeal and procto- 

 daeal invaginations now take place, the inner end of the stomodaeal 



