220 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



other cells are given off from the embryonic rudiment to form the yolk 

 cells. More yolk cells are now set free from the median line of the ental 

 surface of the rudiment, which connect with each other by pseudopod-like 

 processes to form a reticulated sheet between the rudiment and the j^olk. 

 This sheet is the yolk-cell membrane and is interpreted by Leuzinger 

 (1925) as the primary entoderm. There may now be recognized in the 

 embryo an outer layer, about two cells deep, which forms the ectoderm; 

 then a layer of transversely arranged cells, the inner layer; and finally, 

 between the latter and the yolk, the yolk-cell membrane or primary 

 entoderm (Fig. 136A). 



With the longitudinal growth of the embryo and consequent stretch- 

 ing, segmentation first appears when the inner layer divides transversely 

 into metamerically arranged masses. Without the formation of a dis- 

 tinct longitudinal furrow, the segmental masses of the inner layer each 

 separate into two parts. From the mesal margins of the segmented parts 

 (somites) of the inner layer, cells are liberated which pass by way of 

 pseudopod-like plasma bridges into the yolk-cell membrane along the 

 median longitudinal line a position from which they migrate laterally, 

 replacing the original j^olk cells of the membrane and crowding them into 

 the yolk. The replacement cells are regarded by Wiesmann (1925) as 

 mesodermic ; the membrane itself, however, after the substitution for the 

 original cells, is designated as the "secondary entoderm," Although 

 Thomas (1936) stated that the definitive digestive epithelium originates 

 from anterior and posterior mesenteron rudiments, both Hammerschmidt 

 (1910) and Leuzinger and Wiesmann (1926) expressly deny this. 



After the metameric mesoderm plates become two-layered, they 

 acquire coelomic cavities. These coelomic cavities are unusually well 

 developed in Carausius and more or less triangular in cross section, the 

 angles in most of the segments extending out in the form of diverticula. 

 Wiesmann recognizes coelomic sacs in the labrum and in the preantennal, 

 antennal, intercalary, gnathal, thoracic, and abdominal segments 1 to 

 10 (Figs. 13QB,C,D). In Carausius, as in Blatta, the eleventh coelomic 

 sac is but feebly developed. The cavities of the labral and preantennal 

 sacs are evanescent, and that of the intercalary sacs restricted to a small 

 cleft. Except for the last three abdominal sacs the antennal sacs persist 

 after all the others have broken down for the formation of fat bodies and 

 muscles. The median walls of the antennal sac form the aorta; the 

 outer or caudolateral walls, the fat body; the lateral walls, the sheath 

 of the corpora allata. According to Wiesmann the subesophageal body 

 develops from the intercalary sac. In addition to fat and body muscles, 

 splanchnic mesoderm is formed from the coelomic sacs of the labial to the 

 seventh abdominal segments, inclusive. Cardioblasts, from which the 



