596 



ARTHUE WILLET. 



cavity is present as a closed sac, and the amnion itself has 

 completely separated from the rest of the blastoderm, which 

 has, ipso facto, become converted into the serosa. 



At a later stage the amnion enters for a second time, i.e. 

 secondarily, into relations with the serosa, fusing with it near 

 the head end of the embryo. The fused area becomes perfo- 

 rated and the embryo undergoes a process of eversion (revolu- 



Micr 



*» St 10 



Fig. 4. — Egg of Forficula after eversion of embryo, with caudal flexure. 

 Micr. Micropyle. am. Amnion reflected over dorsum, ser. Dorsal organ 

 (remains of serosa). St^. — St.^^. Stigmata. Stbl. Stiuk-gland. (After 

 Heymons.) 



tion of Wheeler, Umrollung of Heymons and others), everting 

 itself through the opening thus produced. 



The secondary fusion of the amnion with the serosa is the 

 initial stage in the involution or retrogressive development of 

 the serosa. By the eversion of the embryo, the amnion 



