1901.] NATURAL SCIENCES OF PHILADELPHIA. 713 



intestine, about 7 mm. anterior to the anus (see fig. 17), where 

 the caeca become more and more shallow and finally disappear 

 altogther, leaving the simple tube of the end intestine, the gonads 

 terminate (cf. fig. 26). 



The Testis. — The testis is a simple sac, the wall consisting of a 

 one-celled layer of large, rather flattened cells with prominent 

 nuclei. In cross section, Plate XLI, fig. 24, the wall of the gonad 

 appears like a membrane, along which the nuclei are placed; in tan- 

 gential section, Plate XLIII, fig. 53, the cells appear elongated, 

 dovetailing into one another, with strong cell walls which are wavy 

 in outline, and granular cytoplasm that stains pink with the haema- 

 toxylin-eosin stain. Each testis has a single duct, figs. 24, 55, T.d., 

 opening on the dorsal surface of the body, very near the side of the 

 rhynchoca'l. The duct passes dorsally from the testis through the 

 inner lougitudinal and circular muscle layers, and then expands into 

 a bulb-like portion with slightly thickened walls, fig. 55, h, and again 

 narrows before opening to the exterior. The duct meets the body 

 epithelium at the surface, and no bending in of the latter has been 

 observed. 



The youngest male cells are found at the margin of the testis, 

 attached by their bases to the gonad walls, figs. 24, 57. The 

 youngest cells are the largest — .023 mm. long, .005 mm. wide. 

 They are rather pear-shaped cells, fig. 57, with the distal end rounded 

 and the basal end prolonged into a slender stalk. The nuclei, 

 N., are large, with a prominent chromatin reticulum and one 

 nucleolus, n. The cytoplasm is finely granular and no cell mem- 

 branes are present. From their size it is probable that these cells 

 are spermatogonia. They form a layer, one or two deep, around 

 the periphery of the testis, and are occasionally found far in toward 

 the middle. 



On the median side of the spermatogonia comes a zone of smaller 

 cells, arranged in radial rows. These are probably spermatocytes. 

 Many of them are in division stages, and the tiny spindles are very 

 distinct, but are too minute for any exact study. 



In the innermost part of the testis the spermatozoa, Plate XLI, 

 fig. 24, Spz., are found, and near them very small cells that are no 

 doubt the spermatids. The spermatozoa are about .06 mm. long, 

 and three parts may be distinguished in them, the head, the middle 

 piece and the tail, Plate XLIII, figs. 58, 50. The head is longer 



