186 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



those of the endsac, and the transition at the mes-ectal line is, as I have 

 said, abrupt, there being no cells »f intermediate character (Fig. 51, In. 

 mes-ec). The cells of this ectodermic part differ considerably in size and 

 shape, although not essentially in structure. Figures 54 to 57 (Plate 6) 

 are all drawn from the same series of sections, and with the same ampli- 

 fication, but from different regions in the wall of the ectodermic sac ; 

 they fairly illustrate the variation in the size and the shape of the cells. 

 The cells from the dorso-median wall (Fig. 56) are more columnar, and 

 the nuclei are relatively farther from the basal end of the cell than in 

 other regions. Those from the ventral face (Fig. 55) are much smaller 

 and show fewer striations. The cytoplasm in all these regions is granu- 

 lar, more densely so than in the endsac, and in all but the ventral wall 

 (Fig. 55) the granules are arranged in rows perpendicular to the base- 

 ment membrane and separated by clearer areas of periplasm. This 

 striation is found both in the basal and free ends of the cell. In some 

 cases the clear area between rows of granules is expanded to form large 

 vacuoles (Fig. 56, vac). The nuclei are bounded by a definite nuclear 

 membrane and have a coarse chromatic network, which stains deeply 

 in hematoxylin. Where the strands of this network cross, there are 

 formed chromatic masses of greater density ; these are usually triangular 

 in section (Figs. 54, 57, and 58). Nucleoli are found in some cells 

 (Fig. 55). 



Mitotic phenomena, though not abundant in these cells, are now and 

 then met with. One such cell in pi'ocess of division is shown in Figure 

 57. The plane of division is in all cases perpendicular to the basement 

 membrane, so that the tissue remains only one layer thick. The rarity 

 of these indications of cell division, in spite of the considerable increase 

 in the size of the gland in the beginning of the second larval stage, is 

 perhaps an indication that the cell increase in the organ is more rapid 

 in the latter part of the first larval period than in the earlier part, at 

 which time my material was killed. 



The basement membrane is seen in all cases. Under high powers it 

 has an appreciable thickness (Fig. 58, Plate 6, mb. ba.). I have seen 

 no trace of nuclei in it to indicate an endothelial nature. 



All the cells of the ectodermic sac agree in having a vertically striate 

 cuticula on the free face (Figs. 54-58, eta.). Under high magnification 

 this cuticula is seen to be made up of bundles of rods or strings of gran- 

 ules grouped into the shape of an hour-glass, the clear spaces between 

 these bundles having in section a lenticular shape (Fig. 58, eta.). This 

 cuticula agrees in a measure with that in the walls of the labyrinth of 



