120 GEORGO ORIHAY SHINJI 



that follow the wall of the proctodaeum gradually becomes thin 

 and membranous, while the midgut increases in size (fig. 130). 

 After hatching, however, the cephalic portion of the midgut 

 becomes enlarged and finally assumes a shape very much re- 

 sembling the human stomach, while the stomodaeum remains a 

 slender tube throughout (figs. 56 and 132). A greater portion 

 of the midgut of certain coccids, especially that of Aspidiotus 

 nerii, was thought by Mecznikow to be derived from the elonga- 

 tion of the proctodaeal invagination, because, as he stated, the 

 alimentary canal of coccids is very short. In the case of Icerya 

 it was a very difficult matter to distinguish the rudiments of the 

 midgut in the early stages, and consequently I might have also 

 arrived at the same conclusion as Mecznikow did, had I not also 

 studied its development in the mealy bug. 



In all insects both the fore- and hindgut are derived from 

 ectoderm, according to different investigators, with the exception 

 of Diptera, in which, according to Voeltzkow ('88) and Graber, 

 they are of mesodermal origin. 



In the aphids, which are closely related to the coccids, the 

 alimentary system is, however, derived from a different source, 

 according to investigators. The fore and hindguts are here also 

 formed from the ectodermal invaginations. The midgut is 

 claimed by Will ('88) to come from yolk cells, but according to 

 Witlaczil ('84) the same organ is formed from the proctodaeal 

 and stomodaeal invaginations and therefore is strictly ectodermal 

 in origin. 



In the majority of insects, however, the midgut is formed from 

 the anterior and posterior rudiments. These rudiments were 

 frequently spoken of as entoderm cells, but more often claimed 

 to be ectodermal in nature. The supposed entoderm cells in the 

 case of the honey-bee are, according to Nelson's research, derived 

 from anterior and posterior portions of the blastodermic tube 

 which is in direct continuation with the side plate or ectoderm. 



Thus, the point of interest in the formation of the midgut in 

 coccids is this : the midgut is entirely derived from its rudiments, 

 the entoderm cells, situated at the posterior end of the embryo. 

 This is, so far as my knowledge goes, a new type of the midgut 

 formation, never recorded in the case of the class Insecta. 



