278 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



with the developing proctodaeum (Fig. 211, mge), develops into the 

 posterior mesenteron rudiment. As with the anterior mesenteron rudi- 

 ment, two ribbons of cells, here anteriorly directed, grow out from the 

 posterior rudiment and, fusing with the anterior ribbons, later spread 

 around the yolk to form the mid-gut epithelium. In the main, the 

 observations of Karawaiew (1893) regarding the development of the 

 mid-gut agree with those of Seidel (1924). In Pyrrhocoris the middle 

 longitudinal section (middle strand) of the inner layer breaks down. In 

 this insect the inner layer may thus be regarded as composed of a middle 

 section (middle strand), anterior and posterior mesenteron rudiments, and 

 two lateral mesoderm strands. 



mge 



mge 



Fig. 211. — Pyrrhocoris. 

 segments, (gc) Germ cells. 

 (neur) Neuroblasts. 



proct 



A B 



Sagittal sections. A, fifth (v) and sixth (vi) abdominal 

 B, proctodaeUm {proct). {mge) Mid-gut epithelial rudiment. 



At the time the mesenteron ribbons are approaching each other in 

 Pyrrhocoris and shortly before the differentiation of the lateral bands of 

 the mesoderm, one may observe the germ cells (Fig. 211 A) emerging 

 dorsally from the mesoderm in the intersegmental region between the 

 first to the eighth segments. Prior to this period the germ cells had not 

 become sufficiently differentiated to be distinguished from adjacent meso- 

 derm cells. The germ cells near the median abdominal segments are 

 recognizable as such a little earlier than those of either the anterior or the 

 posterior segments, a fact that, in the opinion of Seidel (1924), indicates 

 that the germ cells were differentiated before the formation of the germ 

 layers. On the inner (dorsal) side of the somatic mesoderm the meta- 

 meric genital ridges develop. Each of these ridges soon shows a differ- 

 entiation into an upper, middle, and lower part. The upper part, or end 

 chamber, will form the envelope for the reception of the germ cells; the 

 middle part will form the ovarian tubes or the testicular follicles; and 

 the lower part will form the duct. After blastokinesis the seven genital 

 anlagen on each side in abdominal segments two to eight unite to form 



