704 Laura Florence 



basal plate and parameres, and the slender part has passed along the center 

 of the vesica, where it is surrounded by a sheath composed of slender 

 muscle fibers. This sheath originates as two lateral bundles on the 

 proximal border of the basal plate and is inserted as fine strands on the 

 wall of the vesica at its junction with the penis. 



At the close of copulation the protractor muscles and the body muscles 

 relax, and the coelomic fluid passes back into the body. The vesica penis 

 is drawn to its resting position by the contraction of the muscle sheath of 

 the slender part of the ejaculatory duct, as well as by the contraction 

 of many fine muscle fibers which arc inserted on the surface of its ante- 

 rior half and have their origin in the dorsal anterior border of the basal 

 plate. When at rest these muscle fibers form a thick layer on the anterior 

 region of the basal plate and a thin layer between the vesica penis and 

 the basal plate. Some muscle fibers originate in the ventral body wall 

 between segments 6 and 7 and are inserted in the anterior border of the 

 basal plate, and these by their contraction bring the framework of the 

 apparatus to its resting position. 



The histological structure of the mesodermal organs shows some interest- 

 ing features. The testes are surrounded by a three-layered wall — an 

 inner slender epithelium, a very fine basement membrane, and a peritoneal 

 wall in which there is no pigment. Fat bodies are closely apposed to the 

 dorsal surface of the testes, and among them, as also in the peritoneal 

 wall, tracheoles are very numerous. The contents of the testes consist 

 of cells and developed spermatozoa, which for the most part he in clusters 

 of from six to twelve individuals. This is similar to the finding of Landois 

 (1865 a ; 53) in Pediculus, and is common in insects. Each spermatozoon 

 has a rod-shaped nucleus in the head, which takes the hematoxylin stain 

 so intensely as to appear black. Anterior to the nucleus can be distin- 

 guished a small area of cytoplasm staining a bright pink with eosin just 

 as the tail stains. No middle piece can be distinguished. These clusters 

 of spermatozoa are in that half of each testis which lies next to the vas 

 deferens, and appear to rest in a matrix of nutritive cells with very pale- 

 staining nuclei. The remainder of each testis is filled with cells typical 

 of the different zones of development. At the base there is a very small 

 cluster of spc^i'matogonia, followed l^y spermatocytes of both orders in 

 process of division and reduction, and then a small section of spermatids. 



