No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 13 



situated at the inner ends of the cells so that masses of 

 enucleate protoplasm are left at the surface. Posteriorly the 

 organ is linked to the embryo by means of a few flattened 

 cells. In the section two of these cells are seen at z differing 

 in no wise from the serosal elements (j-.) in front and on either 

 side of the organ ; the upper cell passes directly into the serosa 

 covering the embryo, while the lower abuts on the cells that 

 form the transition from the ectoderm to the amnion. The 

 ectodermal layer of the embryo {cc) is nearly as thick as the 

 indusium and of similar cytological structure. The begin- 

 ning of the stomodaeal invagination is shown at o. 



The next section figured (Fig. 17 PI. II) is from an indusium 

 in a somewhat younger stage than that represented in surface 

 view in Fig. 2. Being transverse the section shows an evenly 

 convex outer surface, continuous with the surface of the serosa 

 (s?) enveloping the yolk. The cell-contours are still visible 

 and show that the cells constituting the median portion of the 

 organ are polygonal. The nuclei of these elements are spherical 

 or oval and contain one, or more rarely, two nucleoli besides the 

 usual chromosomes. In the peripheral ring-shaped thickening 

 the cells (c/.) are larger and pyramidal or fusiform in outline, 

 while their nuclei differ in no wise from the nuclei of the 

 median cells. The serosal cells stain more deeply than the 

 cells of the organ, as may be seen at s where a single cell 

 overlaps the edge of the disk. This depth of color is appar- 

 ently purely optical, being due to the greater size and flatness 

 of the serosal nuclei. The walls of both the small polygonal 

 and larger pyramidal elements fade aw^ay towards the surface, 

 where the bodies of the different cells become confluent 

 to form a homogeneous mass. 



In this surface-mass of protoplasm which takes the normal 

 pink stain in borax carmine, are to be found several of the 

 peculiar nuclei, mentioned above as distinctly discernible from 

 the surface (Fig. 16). They differ markedly in structure and 

 appearance from the normal nuclei in the inner portions of the 

 indusium as will be seen by comparing the cells of Fig. 24 

 with those in Fig. 23, both of which figures were drawn with 

 a high power. The normal cells (Fig. 23) have spherical or 



