SIPHONAPTERA AND DIPTERA 



365 



disappears altogether. The serosa continues unbroken throughout the 

 development of the embryo until the ninety-second hour, when the larva 

 breaks through its protective coverings and emerges from the egg. The 

 nodules indicating the position of the nuclei mentioned above are large 

 at first but they gradually flatten down into mere oval thickenings in the 



S'^ 



^.-l\ 







Fig. 319. — Sciara. Cross section of germ band, (am) Amnion, (ect) Ectoderm, {il) 



Inner layer. 



serosa membrane. Figures 317 and 318 show them just after the serosa 

 is formed. Figure 326 shows them at an advanced stage of development 

 at the eighty-fourth hour. At no time does the serosa form simply 

 part of the dorsal body wall. It is completed at an early hour and 

 remains a separate and distinct membrane until ruptured in the process 

 of hatching. 



Soon after the fifteenth hour a 

 depression appears on the ventral 

 side that is caused by a layer of 

 cells, U-shaped in cross section, 

 pushing into the yolk along the 

 mid-longitudinal line (Fig. 318). 

 The depression extends the full 

 length of the embryo, although it 

 does not appear simultaneously 

 throughout its extent (Fig. 321). 

 The lateral portions of the embryo 

 separated by the groove come 

 together and fuse, obliterating the 

 groove and pinching off the 

 U-shaped ridge of cells, which then 

 forms an imperfect gastrular tube, 

 the lumen of which is never very distinct except at the ends (Fig. 318). 

 The lumen soon disappears, leaving a mass of cells lying in an irregular 

 elongate heap along the median line on the inside (Fig. 319). 



The heap soon flattens out into a plate of columnar cells nearly as wide 

 as the embryo itself (Fig. 322). 



am 

 Fig. 320. — Sciara. Cross section of germ 

 band at caudal end. (am) Amnion, (ect) 

 Ectoderm, (gc) Germ cells, (il) Inner 

 layer. 



