FORMATION OF THE BODY-CAVITY 563 



In special cases (Apis, Musca) this occurrence seems not to take place. The 

 yolk-cells are still, after the complete formation of the mid-intestine, to be 

 recognized in the yolk-remnants filling the interior of the same, and gradually 

 become absorbed. 



k. Farther development of the mesoderm. Formation of the body- 

 cavity 



We have seen that by means of an invagination extending through- 

 out the entire length of the primitive band a layer of cells is produced 

 which soon spreads out on the inner side of the band and thus 

 forms a second lower (inner) layer (Fig. 539, C). From this inner 

 layer is separated at the anterior and posterior ends of the primitive 

 band, the endoderm, which lies in direct contact with the invagiua- 

 tions of the proctodaeum and stomodseum. The remainder, by far 

 the most extensive part of the inner layer, is the mesoderm. 



The mesoderm now becomes divided into two lateral streaks 

 (mesodermal streaks), by the withdrawal of its cells from the 

 median line (Fig. 539, D). This withdrawal is not, however, 

 always a complete one. In the free median space thus formed, the 

 yolk often forms the so-called median yolk-ridge. Segmentally ar- 

 ranged cavities soon appear in the lateral region of the mesoderm 

 (the primitive segmental cavities), and the bordering mesoderm- 

 cells arrange themselves in the form of an epithelium, and con- 

 stitute the wall of the primitive segments or coaloni-sac. (Kor- 

 schelt and Heider). 



The primitive segmental cavities in general arise through a split in the meso- 

 derm. In Phyllodromia, according to Heymons, the primitive segments are very 

 extensive. The mesoderm, at the time of the formation of the rudiments of 

 the appendages, is raised with the ectoderm from the surface of the yolk, and 

 in this way there arise in each segment cavities, which, since they are sur- 

 rounded by mesodermal elements, become the closed ccelom-sacs (Fig. 540, 

 c, c', c"). 



The coelom-sacs differ in different groups. They are largest in Orthoptera 

 (Phyllodromia), where they take up almost all the cell material of the mesoderm 

 in their formation, and exhibit certain conditions recalling those of Peripatus. 

 The very large primitive segmental cavities, which in Orthoptera also extend 

 into the rudiments of the appendages (Fig. 540, B, ex), in their later stages are, 

 through the formation of a constriction, divided into a dorsal and a ventral half 

 (Fig. 540, J3, c', c"). The ventral portions of these cavities extending into the 

 extremities soon disappear, while the cells of their walls lose their epithelial 

 nature, and group themselves irregularly into a sort of mesenchym. In this 

 tissue, then, arises, partly through a separation among its cells, partly through 

 the elevation of the same from the upper surface of the yolk, the definite ?>.///- 

 cavity. The dorsal portions of the primitive segmental cavities remain unchanged 

 a longer time in order to play a role in the formation of the intestinal muscular 

 layer, of the heart, pericardial septum, and sexual organs. 



