No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 115 



strikes only the lowermost germ-cell of the cluster in the third 

 abdominal segment {gdz) and in the fourth segment passes 

 completely under the cluster. Even at this time certain meso- 

 derm-cells, the future epithelial elements {ep) begin to flatten 

 out and apply themselves to the surfaces of the germ-cells. 



The exact relations of the primitive germ-cells to the walls 

 of the somite are readily seen in transverse section (Fig. 53). 

 The inner face of the triangular somite is applied to the surface 

 of the yolk and besides giving rise to the germ-cells will ulti- 

 mately form the splanchnic, or visceral layer. The remainder 

 of the coelomic wall is somatic, or parietal, and is converted 

 into fat-body and musculature. The heart arises where the 

 outer edge of the splanchnic passes into the somatic layer. 

 In the section figured the entoderm is still wanting on the left 

 side, whereas on the right side a single cell {eii) of the right 

 posterior band has already reached the segment. Similar ine- 

 qualities in the rate of growth of the entoderm-bands are by 

 no means infrequent. 



In this stage some of the primitive germ-cells show a 

 tendency to leave the wall of the somite and to drop into the 

 coelomic cavity. This is distinctly seen in Fig. 53. These 

 cells sometimes enlarge considerably, become vacuolated and 

 take on the appearance of young ova. A cell of this kind, 

 nearly filling the coelomic cavity, is shown in Fig. 55. I do 

 not believe that the cells are loosened from the coelomic wall 

 during the process of sectioning. 



Although the clusters of germ-cells normally occur only in 

 the first to the sixth abdominal segments, in one somewhat older 

 embryo (Stage G) a well developed pair of clusters was 

 found in the tenth segment. One of these is shown in 

 sagittal section in Fig. 56 {gd^'^). It resembles the normal 

 clusters in every particular. The same section shoWs the 

 diverticulum of the tenth somite {in. d.). In the next section 

 laterad to the one figured, the hollow tip of the diverticulum 

 is seen to terminate in the right appendage of the segment. 

 A similar relation of the coelomic diverticula to the append- 

 ages obtains in all the abdominal segments in front of the 

 tenth. 



