The Hog Louse 705 



The remaining mesodermal structures are slender tulDes varying- in 

 diameter. Seen in cross section (Plate LXIII, 3) the inner layer of their 

 wall is composed of epithelial cells resting on an exceedingly fine basement 

 membrane. Outside this is a thin, structureless layer, the true nature 

 of which has not been determined. The anterior half of the ejaculatory 

 duct is sm-rounded by a strong wall of circular muscle fillers, among 

 which are also, toward the posterior end, strands arising at right angles 

 to the duct wall and passing to the outer edge of the circular fibers. Nuttall 

 (1917 a: 307) attributes this strong development of muscle to the force 

 necessary to drive the spermatic fluid down the long, slender part of 

 the duct. 



The epithelial cells lining the vasa deferent ia are small and somewhat 

 flattened, and have a straight surface in the lumen. The anterior sections 

 of the seminal vesicles act as a reservoir for the developed spermatozoa, 

 and there, as in the sections of the vasa deferentia, they can be seen. The 

 epithelium of the region is regular and columnar, and the nuclei, which 

 are circular and have a well-defined nucleolus, lie near the base of the 

 cells, of which the cytoplasm contains many dark-staining granules. 

 Lower in the tubes the cells lose their clearly defined inner borders and 

 appear finely granular, while a secretion which stains a deep pink with 

 eosin surrounds the spermatozoa. This secretion soon takes a definite 

 form and is oval in outline, and, in aippearance, not unlike a cross section 

 of an orange or the illustration of the " Magenscheibe " given by Landois 

 (1865a: PI. IV, fig. 8), and it contains minute vacuoles (Plate LXIV, 6). 

 These suggest spermatophores, but in section no spermatozoa could be 

 seen within them. Still farther along in the vesicles the inner boi'ders of 

 the cells project into the lumina as blunt, thumblike processes, which 

 are slightly pink in sections stained with hematoxylin and eosin, while 

 the remainder of the cell is dark blue. No cell walls are seen and the cells 

 are evidently in active secretion. The clearly defined " spermatophores " 

 now become markedly vacuolated and gradually lose all semblance of a 

 definite form. Probably this secretion acts as a solvent. In the anterior 

 part of the muscular section of the ejaculatory duct the cells are small, 

 but in the posterior part the epithelium is much thickened and has a 

 markedly glandular appearance. From many of the cells of the vesicles 

 and the ejaculatory duct, slender processes project into the canals and 

 even directly into the central mass of secretion, while in some parts of 



