18 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



formation of the mid-gut epithelium, some to disintegrate in the yolk, 

 and some to form the blood cells (bl). It is probable that those which 

 are destined to form the blood cells really arise from the lateral median 

 margins of the coelomic sacs. This liberation of cells takes place in the 

 gnathal head segments as well as in the thorax and abdomen. The genital 

 ridges are not found in the head or the two anterior thoracic segments, 

 and cardioblasts are lacking in some head segments and in the last two 

 abdominal segments. Later the genital ridges degenerate anteriorly and 

 posteriorly, remaining only in those intermediate abdominal segments 

 where the germ cells will later lodge and the gonads develop. 



-vl ■(' 



mge 



ry 



mus- 



ms 





nc 



Fig. 20. — Cross section, (chl) Cardioblasts. (/) Fat body, (mge) Mid-gut epithelial 

 ribbon, (mus) Muscles, (nc) Nerve cord, {splm) Splanchnic mesoderm. 



The Alimentary Canal. — When the posterior end of the embryo has 

 invaginated into the yolk to the maximum extent and the segmentation 

 of the mesoderm has begun, the stomodaeal invagination begins to form 

 in the head at about the level of the future preantennal segment. A 

 little later the beginning of a proctodaeal invagination is to be seen in the 

 last abdominal segment. These two invaginations will give rise to the 

 fore- and hind-gut. The inner layer, as already stated, is made up of 

 three longitudinal strands, the laterals of mesodermic tissue, the middle 

 one regarded by some writers as the secondary entoderm. The extremi- 

 ties of the middle strand, which remain in position when its middle section 

 breaks down, are here designated as the ''mid-gut epithelial rudiments" 

 {i.e., secondary entoderm, or mesenteron rudiments). The stomodaeal 

 and proctodaeal invaginations deepen ; their inner ends impinge upon the 

 mesenteron rudiments (Fig. 19, mge). Through active proliferation each 

 rudiment sends out a pair of ribbons (r) which run lateroventrally of 



