86 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



diately after blastokinesis the inner margins of the two longitudinal 

 entodermal bands approach each other and then fuse below the yolk. 

 The lateral edges then grow dorsad until the yolk is enclosed. Yolk 

 nuclei which have increased in number become attached to the inner side 

 of the entoderm wall and there apparently form a thick cellular layer of 

 cubical cells which is retained during embryonic life. A similar layer is 

 found in the stone fly (Pteronarcys) and also in Periplaneta, although in 

 the latter it degenerates early. Entad of the yolk-cell layer in Eutermes 

 is still another extremely thin layer which appears to represent the much 

 extended limiting membranes of stomodaeum and proctodaeum which 

 have pushed far into the lumen of the mid-gut. 



Tschuproff (1903) has described the development of the mid-gut 

 epithelium in the Odonata. According to her the two extremities of the 

 mid-gut are derived from the stomodaeum and proctodaeum, and the 

 middle section develops from the yolk cells. When yolk segmentation 

 starts, the yolk nuclei form the nuclei of the spherules. Some of the 

 nuclei take no part in yolk segmentation but remain distributed in the 

 yolk, are smaller, and divide mitotically. These are the definitive 

 epithelial cells of the middle section of mid-gut. A lumen is formed in 

 later embryonic life by the arranging of the yolk spherules against the 

 mesodermic layer of the mid-gut. The definitive mid-gut epithelial cells 

 noted above, which were distributed till now between the yolk spherules, 

 migrate to the periphery of the yo\k and against the muscle layer, where 

 they undergo mitotic division. During early postembryonic life these 

 cells give rise to crypts or nests of cells from which the mid-gut is formed, 

 increasing in number, while the yolk spherules gradually are consumed. 

 This concept of mid-gut formation has been modified by recent inves- 

 tigations by the junior author (see Chap. XIV). 



In the stone fly {Pteronarcys proteus), Miller (1939) found that the 

 definitive mid-gut epithelium arises solely from a posterior mesenteron 

 rudiment which is composed of cells that at first form the circumferential 

 walls of the terminal part of the proctodaeum, hence presumably ecto- 

 dermic. The cells of this rudiment spread forward over the yolk side of 

 the entamnion (ental membrane) and increase mitotically, without the 

 formation of proliferating ribbons, until they enclose the entire yolk as a 

 squamous epithelium. A cellular membrane which probably originated 

 from the yolk cells lies entad of the mid-gut epithelium as described by 

 Strindberg for Eutermes. 



COPEOGNATHA, HETEROPTERA, HOMOPTERA 



Fernando (1934) derives the definitive mid-gut epithelium in the 

 psocid Archipsocus fernandi from entodermal cells that wander from the 

 anterior and posterior mesenteron rudiments into the central nutritive 



