76 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



anlage is therefore determined by its fate rather than its origin. If, 

 then, the gastral epithehum of all Metazoa is homologous, so are also 

 their anlagen whatever their mode of formation." 



A brief discussion of the theories tabulated above may be of service. 



la. Primary yolk cells represent the primary entoderm. Gastrulation 

 completed when the so-called ^^ blastoderm' ' is formed. Gastrula furrow a 

 secondary phenomenon. Mid-gut epithelium an ectodermal derivative in 

 most pterygotes. Inner layer wholly mesodermic. 



Heymons (1895a) maintains that in the development of Dermaptera 

 and Orthoptera the epithelium of the mid-gut is formed from ectodermal 

 cells of the blind ends of the stomodaeal and proctodaeal invaginations, 

 confirming views previously expressed by Ganin, Witlaczil, Voeltzkow, 

 and others. Subsequently the results of Heymons have apparently 

 been verified by Friederichs (1906), Lecaillon (1898), Roonwal (1937), 

 and a number of other investigators for some of the Lepidoptera, Coleop- 

 tera, Hemiptera, and Orthoptera. Heymons' conclusion, that the func- 

 tional mid-gut epithelium of pterygotes is of comparatively recent origin 

 and is derived from the ectoderm, whereas the original entoderm is now 

 represented by 3^olk cells constituting a vestigial mesenteron, has been 

 accepted by many though not a majority of workers. In cases where 

 the mid-gut epithelium apparently arises from anterior and posterior 

 rudiments, as in section 16 given below, Heymons (1895a, 1905&) holds 

 that these rudiments develop from the ectodermal (blastoderm'al) region 

 of the germ band before the appearance of the fore- and hind-gut invagina- 

 tions and therefore are ectodermal. 



Among many Annelida, the ectoderm, strictly speaking, arises from 

 two kinds of cleavage cells: entodermal macromeres and entodermal 

 micromeres, both types occurring among primitive Arthropoda (Myria- 

 poda, Thysanura, Odonata) in the form of yolk cells. A sharp line 

 cannot be drawn between macromeres and micromeres, since the former 

 even in early embryonic life are functioning nutritive cells (trophocytes), 

 the latter are regenerative elements. From the micromeres a portion are 

 transformed into new trophocytes (mid-gut epithelium) which after a 

 period of functioning likewise degenerate to be again replaced by new 

 elements (Heymons, 1901). Among the Odonata (Tschuproff) the 

 yolk cells give rise to the middle section of the mid-gut, but the anterior 

 and posterior extremities of the mid-gut arise from small ectodermal 

 regenerative cells. Other investigators who support the view of the 

 ectodermal origin of the mid-gut epithelium but who may not be in agree- 

 ment as to the yolk cells are the following: for beetles, Czerski (1894), 

 Deegener (1900), Friederichs (1906), Lecaillon (1898), and Saling (1907); 

 for Melophagus, Pratt (1900) ; for Mantis, Rabito (1898) ; for Lepidoptera, 

 Rizzi (1912), Schwartze (1899), and Toyama (1902). 



