CHAPTER IX 



MESODERMAL DERIVATIVES 



COELOMIC SACS 



The mesoderm at gastrulation in the Eutracheata, as has already been 

 described, may develop in several ways: (1) by proliferation and immigra- 

 tion from the median longitudinal area of the ventral blastoderm (Fig. 

 AlF) — this type is found among Apterygota, and Orthoptera {Gryllotalpa, 

 Blattella, Periplaneta, Gryllus, etc.) — (2) by growth of the lateral ecto- 

 dermal plates over the median plate as in Gasteroidea, Pieris, Diacrisia, 

 Sphinx, Apis (Figs. 4:ID,E), etc.; (3) by invagination of a median strip 

 of the ventral blastoderm which forms a tube, as in some Coleoptera, 

 Diptera (Fig. 329 A), Chalicodoma, etc. This tube flattens down, 

 obhterating the lumen, and thus forms a solid multilayered mesodermal 

 mass. Following gastrulation, segmentation takes place whereby the 

 mesoderm is divided up into masses corresponding to the future body 

 segments. The transverse segmentation is most distinct in the more 

 primitive pterygotes, especially the Orthoptera. Later a cavity, or 

 lacuna, appears in the right and left halves of most of the m'esodermic 

 segmental rudiments. The lacunae are the coelomic cavities; the 

 mesodermic tissue surrounding them, the coelomic sacs (Figs. 151, coel). 



In most instances the cavities of the coelom are formed secondarily 

 by clefts that appear in the sohd mesoderm. This is the case in Gryllus, 

 Gryllotalpa, Carausius, Hydrophilus, Callandra (oryzae), Siphonaptera, 

 Formica, etc. In other insects the lateral margin of each mesodermic 

 segmental rudiment is folded over, thus enclosing a cavity. This type 

 is found in Locusta, Blattella, Eutermes, Sialis, Gyropus, Diacrisia, 

 Callandra (callosa), etc. 



In the case where the mesoderm is formed as a flattened tube, the 

 cleft, or schizocoele, does not necessarily develop in the place originally 

 occupied by the lumen of the tube. However, Carriere (1890) found 

 that in Chalicodoma the clefts arise precisely where the lumen of the 

 mesoderm tube was located. Heider (1889) described a similar condition 

 in Hydrophilus. If this type of development really occurs in these 

 forms, we have examples, apparently rare among insects, where the 

 coelomic cavities may develop directly into the definitive haemocoele. 

 Here the cavities (enterocoele) of the primitive segments represent 

 paired diverticula of the archenteron (Figs. 39, 40). The inner wall 



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