Dorsal Closure — Endoderm 97 



Sawflies, there may be no rupture of the membranes, 

 but a backward growth of both primitive streak and 

 amnion around the yolk until the latter is enclosed by 

 the ectoderm, which is then itself completely enveloped 

 by the amnion (2, 53). 



In describing many of the organs of the adult insect 

 — the air-tubes, fore-gut and hind-gut for example — 

 it was stated that these are inpushings of the outer 

 skin. We now see that this is no mere figure of 

 speech, but the expression of the simple fact that 

 these organs, though internal in the developed insect, 

 all arise from the outer skin or ectoderm of the 

 embryo, 



Endoderm. — The inner-cell layer of the primitive 

 streak must now be considered. As already stated, 

 it is due to the inpushing of the primitive groove 

 sinking a mass of cells under the ectoderm, beneath 

 which they spread to form a primitive inner layer 

 {endohlasi). Thence are developed the inner and 

 middle embryonic cell-layers — the endoderm and the 

 mesoderm. The endoderm arises from two cell-masses 

 situated at either end of the embryo in the neighbour- 

 hood of the fore-gut and the hind-gut.^ 



Mid-gut. — These cell-masses grow respectively 

 backwards and forwards and become tubular, uniting 

 to form the mid-gut {mesenteron), which ultimately 

 opens into the fore- and hind-guts. Thus the food- 

 canal of the insect is completed. In some cases (Bees) 

 the hind-gut remains shut off from the mid-gut dur- 

 ing the early stages after hatching. 



Mesoderm and Coelom. — The rest of the primi- 

 tive inner layer forms the mesoderm, from which arise 

 the blood-system, muscles, and reproductive organs. 

 By division of the mesoderm into two layers paired 



1 The three embryonic layers, ectoderm, mesoderm, and endoderm, 

 are often known as epiblast, mesoblast, and hypoblast. 



G 



