162 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



extruded material it is not possible to evaluate these results. However, 

 Sturtevant (1929) shows by genetic analysis of Drosophila gynandro- 

 morphs that the presumptive imaginal disks must occupy the same 

 relative positions in the blastoderm as the points where they later make 

 their appearance in the larva. As healing processes are not involved in 

 this case, it seems that a shift in prospective significances such as 

 Schnetter describes for the honeybee does not occur in Drosophila. 



ORGAN FORMATION 



Endoderm. — There are no really pertinent experimental data con- 

 cerning the endoderm. The mid-gut is clearly not the primitive archen- 

 teron. It is formed, practically regenerated, later in development from 

 rudiments. Eastham (1927) and Snodgrass (1935) review the subject 

 from a comparative-morphological standpoint. The principal difficulty 

 arises from the fact that in some insects the lining of the mid-gut arises 

 from mesenteron rudiments carried in on the tips of the stomodaeal and 

 proctodaeal invaginations, whereas in other insects this lining is produced 

 by proliferation from the tips of unilaminar stomodaeal and proctodaeal 

 invaginations. Eastman (1927) and Richards (1932) have suggested 

 that this is only a difference in the time of determination of the func- 

 tional endoderm. Using maps of prospective significance, Richards 

 illustrates the difficulty encountered if we consider the functional 

 endoderm as determined before its growth into the definitive mid-gut 

 in forms in which it arises by proliferation from the unilaminar tips of 

 the stomodaeal and proctodaeal invaginations. He suggests that in 

 such forms it is not determined before this time and that its determina- 

 tion must be a function of the position of the cells concerned. 



Reith (1925) reports that the mid-gut anlagen are practically the 

 only part of the housefly egg capable of development beyond their pro- 

 spective significance. In this species more than half the mid-gut is 

 formed from one end when either the stomodaeal or the proctodaeal 

 invagination is absent. 



Germ Cells. — In certain insects (honeybees, moths, etc.) the gonads 

 and apparently also the germ cells originate in the genital ridge of the 

 splanchnic mesoderm. This type has not been studied experimentally 

 during embryonic stages. In certain other insects the germ cells are 

 segregated at the posterior pole of the egg during cleavage (called "pole 

 cells"). Their further development is more or less independent of the 

 rest of the embryo. 



Hegner (1908, 1911) showed that the elimination of the posterior 

 l)ole from the eggs of chrysomelid beetles, either by pricking and allowing 

 part of the egg contents to flow ovit or by killing with a hot needle, results 

 in an embryo lacking germ cells and possessing certain structural defects. 



