364 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



While the inner layer, which is to be described later, is forming, the 

 embryo lengthens rapidly. The head end reaches the anterior pole of 

 the egg at the twentieth hour and extends to the dorsal side where it 

 develops into two hollow lobes. The posterior extremity in the mean- 

 time pushes under the germ cells and grows into a spiral around the 

 germ cells on the dorsal side. It reaches its extreme length at the 

 twenty-fifth hour (Fig. 321). The lengthening of the posterior extremity 

 in Sciara and in Musca corresponds to the rotation of the embryo of 

 some other insects. There is no rotation about the longitudinal axis in 

 Sciara at any time. 



As the embryo lengthens, the amnion breaks on the ventral side. The 

 anterior portion of the amnion disappears before the twenty-seventh 



Fig. 318. — Sciara. Cross section of 15-houi geim band {am) \inmon. {amf) Amniotic 

 fold, (ect) Ectoderm, (il) Inner layer, (ser) Serosa, (y) Yolk. 



hour, probably being absorbed in the yolk. The posterior end of the 

 amnion remains until about the fiftieth hour. During this time it 

 shortens, and at its inner end where it attaches to the posterior extremity 

 of the embryo it becomes thick. As the posterior end of the embryo 

 uncoils, the thickened amnion increases in size, and shortly before the 

 fiftieth hour it completely disappears probably being absorbed in the 



yolk. 



The serosa at the time the head invaginates is very thick, especially 

 in the anterior end of the egg. It thins out during the next three hours 

 into a membrane with many nodules on its inner surface, indicating the 

 location of the nuclei (Fig. 317). The primary dorsal organ (do), which 

 appears at an early stage before the amniotic folds have closed over the 

 ventral side, is a thick saucer-shaped group of cells on the dorsal side 

 that projects into the yolk. It reaches its greatest size at the fifteenth 

 hour and from then on decreases, until shortly after the thirtieth hour it 



