204 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



eleventh pairs of appendages are most distinct ; the others are rudimentary 

 and transitory, as are also the minute intercalary appendages between the 

 antennae and mandibles. 



A thin membrane associated with distinctive cells, for which the term 

 "ental membrane" was proposed by Miller, develops and covers the 

 entire upper surface of the embryo, separating it from the yolk. It is 

 attached to the marginal ectoderm, but its mode of origin is uncertain. 

 During dorsal closure it is carried upward by the advancing body wall and 

 encloses the yolk; it serves as a base for the spread of the cells of the 

 definitive mid-gut epithelium. 



The fore-gut and hind-gut arise from the stomodaeum and procto- 

 daeum, respectively. The definitive mid-gut epithelium evidently arises 

 solely from a posterior mid-gut rudiment. The latter is composed of 

 cells that at first form the circumferential walls of the terminal part of the 

 proctodaeum and that become differentiated only after the proctodaeum 

 is well formed. Similar cells give rise to the three Malpighian tubes. 

 The apical closure of the proctodaeum ruptures, and the enlarged mid-gut 

 rudiment cells are everted to form a discoid, unilayered plate. The cells 

 of this rudiment spread forward over the yolk side of the ental membrane 

 and increase mitotically (but not as proliferating bands) until they 

 enclose the entire yolk as a squamous epithelium with widely spaced 

 nuclei. The epithelial cells become cuboidal by the time hatching occurs, 

 but their definitive columnar form is not attained until the first post- 

 embryonic ecdysis approaches. In the nonfeeding first instar, the mid- 

 gut is lined by a cellular membrane that must originate either from the 

 mid-gut epithelium or from vitellophags; this is evacuated during the first 

 molt and replaced by a true peritrophic membrane in the second instar. 

 Most, if not all, of the vitellophags degenerate before hatching occurs. 

 The usual difficulty is encountered in attempting to assign the mid-gut 

 epithelium to a specific germ layer. Its proctodaeal origin favors the 

 view that it is of ectodermal nature in Pteronarcys, unless the debatable 

 concept of "latent secondary entoderm" is evoked. 



Embryonic development in Pteronarcys, in general, resembles most 

 closely that of the Isoptera; orthopteroid affinities "are also apparent. 



EMBIARIA 



Embia uhrichi de Saussure 

 The metamorphosis of the embiids is of a type intermediate between 

 the gradual and complete, the young resembling the adults in the form of 

 the body, but the wings of the males developing internally. The mem- 

 bers of the group are widely distributed in the warmer parts of the world. 

 Kershaw (1914), from whose paper the following account is taken, states 

 that E. uhrichi feeds chiefly on the inner but dead part of the tree where 



