30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.98 



of the anterior mesenteron rudiment (pi. 2, fig. 23). Because of the 

 extremely long embryonic rudiment that is developed in flea embryos, 

 this anterior depression lies at the anterior pole of the egg. Its anterior 

 margin soon grows out to form a double fold (pi. 6, figs. 53, 54), the 

 anterior amnio-serosal fold. Its inner layer, which is continuous with 

 the ventral plate, will become the anterior part of the amnion ; its outer 

 layer is destined to become the anterior portion of the serosa. This 

 anterior amnio-serosal fold, as it increases in length, gradually extends 

 farther laterally (pi. 6, fig. 52), ventrally, and posteriorly. Because 

 of the greater width of the anterior portion of the germ band (pi. 7, 

 fig s - 55> 57) > due to the presence of the procephalic lobes in that 

 region, the anterior amnio-serosal fold is much wider than the cor- 

 responding posterior fold. Posterior to the cephalic region the anterior 

 fold narrows correspondingly to the width of the germ band. This 

 anterior amnio-serosal fold in flea embryos arises in the same manner 

 as do both the anterior and posterior folds in those insects which have 

 the germ band wholly superficial. 



The posterior of the two primary amnio-serosal fold indentations 

 occurs on the ventral plate immediately posterior to the posterior 

 mesenteron rudiment. Its position lies somewhat around the posterior 

 pole of the egg toward the dorsal surface. At first the posterior amnio- 

 serosal indentation is identical to that of the anterior fold, but this 

 resemblance is transitory. It is true that the posterior fold is gradually 

 extended forward as its anterior counterpart is extended posteriorly, 

 but in the case of the posterior fold the indentation increases in depth 

 until it extends through the yolk about half way to the anterior pole 

 of the egg (pi. 7, figs. 55 to 57). This extensive deepening of the 

 posterior amnio-serosal indentation carries the posterior portion of 

 the germ band deep into the vitellus. This inward movement of the 

 embryo is to be regarded as a partial involution process corresponding 

 to the extreme type which is characteristic of odonate and hemipteran 

 embryos. 



Coincident with the involution of the posterior end of the germ 

 band, the two amnio-serosal folds approach one another (pi. 7, fig. 56), 

 and finally join at the midventral region of the egg. When they 

 come into apposition their intervening walls rupture, and the inner or 

 amniotic layer and the outer or serosal portion of each fuse with the 

 corresponding parts of the other. This fusion occurs during the third 

 day of the embryonic period. In this way there occurs a complete 

 separation of the two embryonic envelopes, producing a double pro- 

 tective covering for the embryo over its ventral surface (pi. 7, fig. 57 ; 



