3i8 DISCOVERY REPORTS 



Structure as the remainder of the cytoplasm. The appearance of this zone of cells 

 suggests considerable secretory activity. 



The nuclei of this zone are of the same size as those found elsewhere in the tubule, 

 but do not possess any prominent amount of chromatin, some indeed being con- 

 spicuous by their lack of it. On the whole, however, they resemble those of the ventral 

 "lens" zone. 



The third zone, beginning immediately above the vacuolated zone, is not very sharply 

 differentiated from the fourth or most dorsal zone, in fact it may not be justifiable to 

 regard these two as constituting separate and distinct zones. They are distinguished 

 from each other by the occurrence in the cells of the fourth zone of spindle-shaped 

 inclusions (Figs. 3, 4, ref.r.) which will be described later. The cells of the third zone 

 (Fig. 3 and Plate XXIV, fig. 4, er.c.) are very similar in size to those of the preceding 

 zone, but show no trace of vacuolation. Their cytoplasm is dense and almost uniformly 

 distributed, being only very slightly denser at the distal tips of the cells than elsewhere. 

 Furthermore, the cytoplasm of the upper cells of this zone is somewhat less dense than 

 that of the lower cells. Owing to their apparent peculiar behaviour I have called the 

 cells of this zone "erupting cells". In some of my preparations the conditions repre- 

 sented in Fig. 4 are seen. Most of the cells of this zone possess a prominent nucleus 

 with a considerable amount of chromatin, usually condensed into a central chromatin 

 knot. From this knot radiate irregular chromatin strands, and further chromatin is 

 deposited on the inner wall of the nuclear membrane. This may be called the usual 

 condition of the resting nucleus of an erupting cell (Fig. 4, n.n.er.c). Other nuclei, far 

 less numerous however, are remarkably deficient in chromatin, having only small 

 irregular deposits placed on a coiled linin thread, or having merely a small central 

 chromatin mass, the remainder of the contents of the nuclear membrane consisting of a 

 substance with a very finely granular appearance (Fig. 4, deg.n.er.c). There can be 

 little doubt that these are phases in the degeneration of the normal nucleus. In several 

 preparations the cytoplasm of one or more cells of this zone had erupted almost com- 

 pletely into the lumen of the tubule. Fig. 4 shows such a cell, the nucleus of which is 

 almost achromatic {deg.n.er.c). Lying in the lumen of the tubule are two nuclei (deg.n.) 

 in a more advanced state of degeneration. The lumen of nearly every tubule examined 

 contained irregular masses (Fig. 3 and Plate XXIV, fig. 4, er.cyt.) having the staining 

 properties of the cytoplasm of these cells, and presumably having had their origin in 

 this eruption. It appears probable, then, that the cells of the third zone regularly erupt 

 their contents into the lumen of the tubule, but although special search has been made 

 no indications of proliferation on the part of neighbouring cells, which would result in 

 regeneration of this part of the tubule, has been observed. 



The fourth, or most dorsal, zone of the cells composing the tubule shows no essential 

 difl^erences from the previous zone, excepting the presence of the rod-like bodies 

 already mentioned. The cells are of the same size as those of the preceding zone, their 

 staining reactions are the same, and the occasional occurrence of nuclei deficient in 

 chromatin in the more ventral cells of this zone (Fig. 4, deg.n.ref.c.) indicates that 



