EPHEMERIDA, ODONATA, PLECOPTERA, ET AL. 209 



invagination is covered on the ventral side by the amnion made up of 

 rounded cells. On the dorsal side are large trophocytes whose plasma 

 still retains the alveolar structure of the original egg contents. The 

 trophocytes correspond to the vitellophags, or yolk cells, of other insects, 

 the alveolar protoplasmic envelope taking the place of the yolk. These 

 cells, which number on an average but 8 to 10, lie in contact with the 

 anterior placental cell mass, presumably acting as intermediaries in 

 conveying nutriment from the placental cells to the embryo. Here and 

 there on the outer side of the amnion certain cells develop by delamination 

 another layer which later separates from the amnion to form a serosa-like 

 envelope. The germ band soon elongates until head and tail nearly meet 

 on the ventral side. Meanwhile serosa cells have pushed in between the 

 trophocytes and the placental follicular wall, wholly separating the 

 former from the latter. It is thus apparent that an interchange of nutri- 

 tive material between the maternal tissue and the trophocytes no longer 

 takes place, the connection between the follicular wall and embryo being 

 established by the embryonic envelopes which henceforth take on the 

 nutritive function. Amnion and serosa are nutritive organs and not 

 protective envelopes as in other insects. In Hemimerus, therefore, these 

 envelopes might be called ''trophamnion" and "trophserosa," as in cer- 

 tain parasitic Hymenoptera. With the increasing nutritive activity of 

 the envelopes the embryo develops rapidly, with a corresponding degener- 

 ation of the placenta cells. The formation of the nervous system, the 

 arrangement of the coelomic sacs, and the development of oenocytes and 

 other structures offer nothing remarkable. 



In Hemimerus the revolution results in the ventral side of the embryo's 

 being turned toward the ventral follicular wall, while the head end of the 

 embryo after complete revolution is directed toward the anterior pole 

 and the caudal end toward the basal end of the maternal follicle. Unlike 

 other insects, however, the envelopes during revolution remain unbroken. 

 The embryonic envelopes after revolution become alveolar and several- 

 layered at both ends of the embryo and may here be designated as the 

 "fetal placenta." 



At the time of the revolution of the embryo the ectoderm closes over 

 on the dorsal side. The posterior part of the head remains open for some 

 time after the ectodermal closure of the remainder of the dorsal wall. 

 Here a head vesicle, a diverticular structure peculiar to Hemimerus, is 

 developed from cells of the anterior portion of the amnion and serosa and 

 projects into the placenta. The membranous wall of the vesicle merges 

 with a delicate mesodermal layer that lines a blood sinus which is situated 

 in the posterior part of the head and is connected posteriorly with the 

 dorsal blood vessel. At this stage the blood vessel is already provided 

 with muscles so that blood conceivably may circulate through these parts 



