1 88 MARGARET R. HAASE. 



In the histological structure of these pits we define three 

 regions; the neck or transitory region (t), the sensory region (s), 

 and the glandular region (g) (Figs. IV, V, and VII). All the 

 components of the pits, with the single exception of the muscle 

 fibers (Figs. IV 7 , VI, and VII, l.m., m.m.}, which are attached to 

 the pits, are considered to be of ectodermal origin. This as- 

 sumption is based upon the fact that Kepner and Taliaferro 

 ('12) in their study of the ciliated pits of Microstoma caudatum, 

 clearly showed that in a newly-forming individual, both the 

 glandular and sensory cells developed as modifications of an 

 invagination of the general body epithelium or ectoderm (Figs. 

 IV, V, and VII, g.b.e.}. The pits of Prorhynchus stagnalis 

 measure from twenty-five to thirty-five microns in length and 

 from twelve to fifteen microns in width, (excluding the glandular 

 cell, which is about two thirds of the length of the pit proper), 

 being narrower near the mouth in the fixed condition on a slide 

 than at the fundus of the pit. This latter condition, that of the 

 narrowness of the mouth of the pit, is in sharp contrast to the 

 contour of the functioning pit. 



With the exception of the glandular region, the ciliated pit is, 

 like the rest of the body epithelium, a syncytium, with its nuclei 

 specialized to conform to their respective functions. The 

 presence of a glandular cell makes even stronger the belief that 

 the pit is a gustatory organ. 



The nuclei of the ciliated, transitory epithelium are eight in 

 number, a double row of four nuclei, one row being disposed 

 obliquely dorso-ventrally, forming a part of the lateral wall of the 

 pit, while the other row forms a similar mesial wall. These 

 nuclei have an elongated, ovate contour with the axis of each 

 nucleus in a plane that is at right angles to the axis of the pit, and 

 bent to conform to the curved wall of the pit (Figs. IV, V, VI, 

 and VII, /). They are densely granular, and the aspect of the 

 possible fifth pair of nuclei nearest the mouth of the pit gradually 

 fade into the appearance of nuclei of the general body epithelium. 



There are also two rows ot sensory nuclei, four in each row, and 

 placed in the same plane as the nuclei of the transitory cells 

 (Figs. IV, V, VI, and VII, 5). On the inner walls of the pit next 

 to the sensory nuclei are two rather densely granular ridges, 



