322 INSECTA. 



which appear at a comparatively late stage and run out from the definitive 

 body-cavity. 



The mesoderm -bands, which become separated, unite again later, the mesen- 

 chyme-cells from the one side becoming closely applied to those from the other 

 in the median line. After the median yolk-ridge degenerates, a cell-accumula- 

 tion often forms here (Fig. 158 E) ; this extends below the rudiment of the 

 ventral chain of ganglia and owes its origin to the mesenchyme-cells. It is this 

 cell-strand which has been called by Nusbaum (No. 57) the chorda of the 

 Insecta. It is finally used up in the formation of connective and other 

 mesodermal tissues. 



The permanent body-cavity of the Insecta arises quite independ- 

 ently of the coelomic cavities, by a separation of the germ-band 

 from the yolk (Butschli, No. 11, Fig. 158 F, I). It appears to be 

 bounded on the one hand by the surface of the food-yolk and on the 

 other by the irregularly arranged mesenchyme-cells. We can at first 

 distinguish in section three distinct subdivisions of the body-cavity 

 (Hydrophilus, Heidbr) : a median space and two large paired, lateral 

 spaces which unite later with one another, and with other lacunae 

 which have arisen by the shifting apart of the mesenchyme-cells 

 {e.g., in the limbs). We may trace back the spaces of the permanent 

 body-cavity, as in Peripatus (p. 201), to the primary body-cavity or 

 cleavage-cavity. It becomes apparent as a series of lacunae in the 

 mesenchyme and everywhere shows the character of a pseudocoele 

 (cf. Introduction, Vol. i., p. 11). 



In later stages of embryonic development, the coelomic sac's and 

 the permanent body-cavity enter into communication (Fig. 167 A, 

 us, Ih). The consecutive coelomic sacs first fuse together through 

 the degeneration of the transverse dissepiments that separated them ; 

 a slit then opens in the median wall of the coelomic sacs and 

 connects their lumina with the permanent body-cavity. In the later 

 transformations undergone by the wall of the coelomic sacs, the 

 latter can no longer be recognised as separate sections of the whole 

 body-cavity. 



8. The Formation of Organs. 

 A. Outer Integument. 



The hypodermis arises by direct transformation from the cells of 

 the ectoderm. In later embryonic stages, the cuticle of the youngest 

 larval stage is secreted at the surface of the hypodermis. The 

 accessory structures, such as hairs, setae, etc, rise from specially 

 large hypodermal cells (setal mother-cells, Tichomiroff, No. 78). 

 Similar cells (scale mother-cells) give rise in the pupae of the 

 Lepidoptera to the scales of the wings (Semper, No. 126). 



