58 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



OENOCYTES 



These cells, whose function is as yet not definitely known, have 

 been observed in a number of insect forms. In flea embryos they are 

 enormous in size in comparison to the other cells about them (pi. 2, 

 fig. 30). Their nuclei are large, regular, and oval. The origin of the 

 oenocytes has been traced in Melolontha, Lina, and Hydrophilus by 

 Graber (1891). He found that in the embryos of these beetles the 

 oenocytes arise from paired metastigmatic invaginations of the ecto- 

 derm. Wheeler (1892) found similar invaginations in Blatta and 

 Xiphidium, but is of the opinion that they are of minor importance 

 in the differentiation of these cells. He also studied the origin of the 

 oenocytes in a wide variety of embryos including representatives of 

 the Hemiptera, the Ephemerida, the Neuroptera, and the Lepidop- 

 tera. In these forms, metastigmatic invaginations were not found 

 and the oenocytes were observed to arise by simple delamination from 

 the lateral ectoderm. During the present study oenocytes were seen 

 in several stages of development in the flea embryo. As in Blatta and 

 Xiphidium, the oenocytes arise in fleas from the lateral ectoderm of 

 the anterior region of the abdomen, and metastigmatic invaginations 

 are differentiated. When first proliferated, the oenocytes are similar 

 in size and appearance to the other ectodermal cells. They grow 

 rapidly, however, and soon become readily distinguishable. They 

 were observed only in the abdominal region where they are frequently 

 associated with the cells of the fat body within the body cavity. They 

 often lie adjacent to tracheal invaginations. 



OTHER ECTODERMAL DERIVATIVES 



In addition to the ectodermal derivatives which have been discussed 

 already, the ectoderm gives rise to a number of other structures which 

 have not been considered in detail during this study. These include 

 the epidermis (hypodermis), the tentorium and apodemes of the 

 endoskeleton, the corpora allata, the labial (salivary) glands, the silk 

 glands, and the anlagen of the posterior portions of the genital tract. 

 The ectodermal origin of these structures in fleas is in agreement with 

 the work done on other insects. 



In regard to the epidermis it may be added that this layer of the 

 body wall is derived from what remains of the ectoderm in its super- 

 ficial embryonic position after all the invaginations and delaminations 

 to form the internal ectodermal organs have occurred. Furthermore, 

 in flea development, no extra-embryonic dorsal sheet of ectoderm, 

 such as Nelson (1915) describes for Apis, is differentiated. The 



