SIPHON APTERA AND DIPTERA 357 



mesenteron rudiment, the germ cells come to lie on the inner surface 

 of the rudiment. When the posterior portion of the germ band is invo- 

 luted into the yolk, the germ cells, together with the rudiment, are 

 carried along, later to be incorporated into the gonads. 



After the blastoderm has formed, the cells are equally distributed 

 over the surface of the egg. Soon there occurs a concentration of cells 

 toward the ventral mid-line and polar surfaces, forming a ventral thick- 

 ening, the cells of the median dorsal region becoming somewhat flattened, 

 apparently as the result of lateral tension exerted upon them. Although 

 at first there is no sharp line of demarcation between the thinned dorsal 

 area and the thickened ventral blastoderm, along the mid-line near the 

 anterior and posterior extremities of the thickened area rather abrupt 

 swellings soon become evident. These are the first indications of the 

 so-called ''mesenteron rudiments," which at a subsequent stage will 

 form the mid-gut epithelium. They represent the first transformations 

 of the single-layered blastula to a multiple laminated condition. 



The amnion and serosa are formed in the normal manner. At first 

 the posterior amnioserosal indentation is identical with that of the 

 anterior fold. The caudal end of the developing embryo extends over 

 on the dorsal side, extending deep into the yolk and carrying with it the 

 amnioserosal fold in a manner similar to, but in lesser degree than, those 

 of the odonate and hemipteran embryos. There is a fusion of the 

 amnioserosal folds on the third day of the embryonic period, whereby 

 amnion and serosa are completely separated, the former covering the 

 ventral face of the embryo, the latter the entire egg. Later the caudal 

 end of the embryo wdth the amnion still intact comes to lie at the surface, 

 with head and tail ends nearly touching. 



Early on the fourth day the serosa ruptures ventrally and is drawn 

 dorsally to form an indistinct clump of cells which lie for a short time 

 on the dorsal surface of the yolk. This clump of serosal cells is homol- 

 ogous with the secondary dorsal organ but lacks the tubular structure 

 characteristic of this organ. Like a similar structure in Chironomus, it 

 is absorbed in yolk. After the rupture of the serosa the amnion is said 

 to become completely detached from the embryo, forming a complete 

 envelope enclosing the egg contents for a time, when it, too, ruptures 

 along the ventral line and contracts dorsally to be absorbed in the yolk 

 just before the dorsal closure of the embryo. The remains of the amnion 

 before its absorption are called by Kessel the "third dorsal organ" and 

 correspond to the degenerating provisional dorsal wall of those insects 

 in which the secondary dorsal organ assumes a tubular form. 



With the development of the embryonic envelopes the gastrulation 

 process also takes place. At an early stage in the anterior third of the 

 germ band, including a brief section between the anterior mesenteron 



