566 TEXT-BOOK OF ENTOMOLOGY 



Orthoptera. The ventral part is here from the very outset replaced by a mesen- 

 chyin. As a result in these forms also no coelomic diverticula occur in the rudi- 

 ments of the extremities. 



The definite body-cavity of insects arises entirely independent of the cceloin 

 cavities, and in fact, as Biitschli showed, through the separation of the primitive 

 band from the yolk (Fig. 539, F, Z). It appears bounded on the one hand 

 by the surface of the yolk, on the other side by the irregularly arranged 

 mesenchym cells. Originally we can in cross-sections distinguish three separate 

 cavities of the definite body-cavity (in Hydrophilus according to Heider), a 

 median and two larger paired lateral ones which later fuse with each other and 

 with wide lacunae (e.g. in the appendages) arising by the separation of the 

 mesenchym cells. We refer the compartments of the definite body-cavity, as 

 in Peripatus, to the primary body-cavity or segmentation-cavity. They are 

 only lacunae in the area of the mesenchym, and throughout bear the character 

 of a pseudocosl. 



In later stages of embryonic development the coelom-sacs and the definite 

 body-cavity enter into communication with one another (Fig. 523, A, s, lh~). 

 (Korschelt and Heider.) 



Then the hinder coelom-sacs unite through the degeneration of the transverse 

 dissepiments which separate them. After this a fissure opens in the median 

 wall of the coalomic sac, through which its cavity unites with the definite body- 

 cavity. In the subsequent changes which the wall of the ccelom-sacs undergoes, 

 these can be recognised no longer as separate divisions of the whole body-cavity. 



I. Formation of organs 



The nervous system. As we have already seen (p. 554), the rudi- 

 ments of the ventral nervous cord arise, after the gastrnla invagina- 

 tion is completed, as two ectodermal thickenings situated on each 

 side of the median line, the so-called primitive rolls or strips 

 (Fig. 528, s), which extend from the centre of the procephalic lobes 

 of the head to the last segment, enclosing between them the single 

 median "primitive groove" (Fig. 539, C, pr, and^o). 



Soon after the appearance of the primitive strips, the first traces 

 of segmentation may be detected. The ventral cord is from the 

 first in direct connection and continuous with the brain. From the 

 segmental expansions of the primitive strip arise the ventral nervous 

 ganglia, and from the intersegmental constrictions are developed 

 the paired longitudinal commissures. 



Transverse sections of the ectoderm in the region of the primitive 

 strips (Figs. 539, C, and 517) show several layers of cells. Of 

 these cellular layers the deeper ones afterwards, by a kind of 

 delamination, separate from the superficial ones and form the 

 "lateral cords," i.e. the germs of the longitudinal cords of the ven- 

 tral ganglionic cord. Meanwhile the primitive groove (pr) deepens 

 and forms an invagination extending between the lateral cords. The 

 cells at the bottom of this invagination form the so-called "median 



