28 WHEELER. [Vol. VIII. 



of the cephalic end fuses with the overlying inner indusium. 

 A rent appears in this fused portion of the envelopes and 

 through it the head is soon seen protruding. Gradually more 

 of the body is pushed through the orifice, first the mouth parts, 

 then the thoracic legs and finally the abdominal segments, till 

 the whole embryo comes to lie free on the surface of the yolk 

 in the space between the inner indusium and its cuticle. The 

 amnion and inner indusium, which during the evagination of 

 the embryo have remained united at the edges of the rent 

 are folded over the pleural region of the embryo onto the yolk. 

 The two envelopes now form but a single layer enclosing the 

 yolk like a bag. The inner indusium is united to the edges of 

 the amnion and these in turn are united to the pleural edges of 

 the embryo, with the ectoderm of which the amniotic cells are 

 continuous. The small size of the amniotic cells as compared 

 with the huge flattened elements of the inner indusium enables 

 one readily to distinguish the limits of the two envelopes. 



During its evagination from the cavity of the amnion the 

 embryo gradually passes around the lower pole of the ^^% 

 head first and begins to ascend the convex ventral surface. An 

 embryo freed from all its envelopes except the two that take 

 part in revolution is represented in Fig. 8, in the very act of 

 turning the lower pole. The amnion and inner indusium are 

 folded back over the yolk, the former {am) characterized by its 

 small rounded nuclei, the latter {sr) by its large flat elements. 

 The line of juncture of the amnion with the body of the 

 embryo is marked by a denser aggregation of nuclei. The 

 ventral flexure still persists on the dorsal surface. 



The cavity of the amnion contains a quantity of serum-like 

 liquid, which during the evagination of the embryo is poured 

 into the space separating the inner indusium from its cuticle. 

 This liquid collecting at the lower pole, may function as a 

 lubricant and cushion, and thus facilitate the movements of the 

 germ-band. In hardened specimens it is found as a gran- 

 ular magma enveloping the appendages. It is not shown in 

 Fig. 8. 



In many respects the embryo in Stage H has advanced con- 

 siderably beyond that represented in Fig. 7. In the head, the 



