318 INSECTA. 



7. Further development of the Mesoderm. 

 Development of the Body-cavity. 



We have seen (p. 271) that an invagination running along 

 the "whole length of the germ-band gives rise to a layer of cells, 

 which soon extends on the inner side of the germ-band and so forms 

 a second, lower layer (Fig. 158 C). From this layer, at the anterior 

 and posterior extremities of the gerin-band, the entoderm becomes 

 separated and becomes closely applied to the stomodaeal and procto- 

 daeal invaginations which have meantime arisen. The remaining 

 and by far the largest part of the lower layer may, from this stage 

 onward, be considered as mesoderm. 



An arrangement of the latter into two lateral bands (mesoderm- 

 bands) now takes place, its cells "withdrawing more and more from 

 the median line (Fig. 158). This withdrawal from the median 

 line is, however, not complete. Into the space between the two 

 mesoderm-bands the yolk often thrusts itself, giving rise to the so- 

 called median ridge. Segmental cavities (cavities of the primitive 

 segments, tts) now appear in the lateral parts of the mesoderm, and 

 the mesoderm-cells become arranged as an epithelium round these 

 cavities and form the wall of the primitive segments or coelom-sacs. 



The cavities of the primitive segments arise, as a rule, by a splitting of the 

 mesoderm. Heider (Xo. 38) thought that in the case of Eydrophilus he had 

 convinced himself that they arose merely by the widening of a slit, which was 

 already recognisable at an earlier stage between the two layers of the mesoderm, 

 and which could be traced back to the lumen of the archenteron compressed 

 dorso-ventrally. Graber (Xo. 30), however, in his more recent investigations 

 on this point, was not able to satisfy himself of the persistence of these slits. 

 On the other hand, Heider's view has been confirmed by Carriere (Xo. 13) 

 in the case of Chalkodoma. These observations afford support to the view first 

 adopted by 0. and R. Hertwig that the cavities of the primitive segments 

 in the Insecta represent paired diverticula of the archenteron. 



The large primitive segments of Phyllodromia arise in a different manner 

 from those of Hydrophihis. The mesoderm of the germ-band is here at first 

 only a single layer of cells. This simple layer, as the limb-rudiments develop, 

 separates with the ectoderm from the surface of the food-yolk, and cavities thus 

 arise in every segment, these cavities, surrounded by mesoderm -elements, 

 becoming the closed coelomic sacs (Heymons, Xo. 43). 



The parts of the mesoderm lying laterally in the germ-band are used in the 

 formation of the primitive segments (Fig. 158 D and E). Xot all the meso- 

 dermal elements, however, enter into their formation. Some of the mesoderm- 

 cells which lie nearer the median line always remain distinct (cf. Fig. 157 A, m). 

 The greater the size of the primitive segments, the smaller is this remainder, 

 and via versd. These elements are irregularly arranged and represent a kind 

 of mesenchyme. 



It was pointed out by Heider (Xo. 38), and recently by Graber (Xo. 30), 



